Bosch Hydromax brake booster info

[CarolinaHD]

Information/discussion thread for Bosch hydroboost systems found on medium duty trucks, buses and RVs. The most common vehicle they are used on are the 2002-2009 c4500-6500 gmc and chevy top kick and kodiak. I have included all the documents, pics and websites that I have found on my search

https://www.powerbrakeservice.net/bosch-hydromax.html

http://www.powerbrake-drivetrain.com/hydromax/

Ordered my replacement pedal grommet from here, good prompt service:
https://automotivebrakeandclutch.com/?s=Grommet

Below is some info and pics from the technical manual.

Basic Operation of Hydro-MaxTM Booster Assembly and
Master Cylinder:

Reference Figure 6 for the following electrical components of the booster and master cylinder.

Backup Pump: The Hydro-Max hydraulic booster has a backup pump which will provide hydraulic boost at a reduced rate if the normal source of fluid is interrupted (refer to Figure 3). The signal for operation of the backup pump comes from the flow switch. If normal flow is interrupted, the flow switch will close and activate the relay, which will turn on the backup pump. The backup pump is available for 12 and 24-volt systems. The 12-volt backup pump can draw a steady state maximum of 55 amps at a power steering fluid temperature of 1000 F. The 24-volt backup pump can have a steady state maximum draw of 27.5 amps at 1000 F.

Relay: An optional relay is available for attachment to the booster. The relay connects to a sealed and mechanically latched wiring connector. The function of the relay is to provide current to the backup pump when triggered by the flow switch.

Flow Switch: The function of the flow switch is to activate the relay when normal hydraulic power source fluid flow is interrupted, turning on the backup pump. The flow switch has two terminals (A and B). A is positive. B is connected to the booster housing and can be used as an optional harness ground. The flow switch itself is grounded through the booster housing. A sealed and mechanically latched connector connects the flow switch to the backup pump relay circuitry.

Differential Pressure Switch: The differential pressure switch reacts to a loss of master cylinder hydraulic pressure in either side of the split hydraulic system and can illuminate a warning light at the instrument panel. When pressure is lost in either the primary or secondary circuit of the master cylinder, the switch closes. The electrical switch will remain closed until the malfunction is corrected. When both systems develop normal pressure, the switch will return to center and open the electrical switch circuit. A mechanically latched connector is attached to the switch. The switch has two tabs on a common terminal (positive) and is grounded through the master cylinder.

Fluid Level Switch: When the fluid reaches a predetermined level (low fluid level), the switch closes and can illuminate a warning light at the instrument panel. A sealed and mechanically latched connector is attached. The switch has two terminals, positive and negative, which are interchangeable.

Chassis Ground: The booster and master cylinder assembly must be provided with a ground path that will carry the maximum current of the backup pump.

Pics:
Manual cover
Flow diagram
Screenshot from manual
Parts diagram
2003-2009 c4500-c500 typical parts diagram

[CarolinaHD]

1. Bosch technical manual, too large to upload to the site. Describes how it works

https://www.irv2.com/forums/f22/bosc...ual-75711.html

2. Bosch service manual

https://nc4x4.com/forum/threads/rand...7#post-1751263

3. Bepco brake parts catalog

https://www.bepco.biz/products/hydraulic-brake-systems.

4. Rebuild/Parts diagram

http://67-72chevytrucks.com/vboard/a...1&d=1582081769

5. Cheapest replacement backup pump I could find

https://www.dbelectrical.com/product...SABEgLKJfD_BwE


6. Brake pedal travel calculator, calculates the amount of pedal loss or gained when switching brake master cylinder bore sizes.

https://brakepower.com/help_abc_29_PTC_t.htm

[CarolinaHD]

For my 1967 c50 292, I added power steering with brackets from ebay, I'm using all factory drums on the 4 corners, new nickel copper brake lines, new dorman rubber brake lines, relined shoes and utilizing 2 of the factory brass brake blocks for the new split system master cylinder. I opted not to use a proportioning valve because it's all drums. You will need some 10lb residual pressure valves for each line. Google for more info about this.

I ordered a brand new complete Hydromax unit for $450 off ebay and it bolted right in. This particular part number didnt bring up much info but I believe it's from a freightliner. I also bought a Delco hy-power style booster from a mid 80s c50 and it bolted right in as well. The main difference between the two is the hy-power booster has 1 high pressure inlet and 2 outlets, high pressure and low. This is similar to the smaller trucks hydroboost where the fluid flows down to the steering box from the booster. The Hydromax system has 1 inlet and 1 low pressure outlet. This is ideal for my truck because it doesn't have power steering.

The inlet size is 1/2" oring tube and I could not find that fitting anywhere. I had to find the factory high pressure line for a 2007 c5500 and have a hydraulic shop cut off the other end for the factory power steering pump. The pump outlet is 3/8" so the shop crimped a 1/2" fitting on the hose and used a 1/2"-3/8" compression adapter with the factory tube.

My plan for hooking up the backup pump is to retrofit the harness from a kodiak/topkick or just wire it to a relay and momentary push button incase of ps pump failure.

I plan on plumbing my residual valves in between the copper and rubber lines right below the master cylinder.

Pics:

[CarolinaHD]

More pics

1st pic shows 1972 school bus with a Midland vacuum booster and dual outlet master cylinder. Pretty rare combo from what I have seen.. This is same booster I had in my 67. You can see the distribution block is nothing more than for the federally mandated brake light switch.

2nd pic shows the factory distribution block on my 67 c50. I'm using 2 of these and plugging the unused holes. 1 for a single line to the rear, and the other to split for the 2 front lines.

3rd pic is the part number for the hydromax pressure hose from gm: 25902725

[Campbel45]

Alex,

Thanks for posting all this! Great information here.


Question, do you think you will need a proportioning valve for the rear brakes? I may be wrong, but I think I recall most of the braking effort is in the rear with these trucks having four brake cylinders for the rear axle.

[CarolinaHD]

No I dont think I will but we'll find out.


EDIT: Yes I am!

[rkn463]

Originally Posted by CarolinaHD

... I also bought a Delco hy-power style booster from a mid 80s c50 and it bolted right in as well. The main difference between the two is the hy-power booster has 1 high pressure inlet and 2 outlets, high pressure and low...

Hi Alex,

It is interesting that your Delco hy-power unit had the two outlets, the one that I got from my '89 donor had just 1 in, 1 out.

I am curious, since you went with the Bosch unit what did you do with the hy-power unit? My (very expensive) rebuilt electric aux pump from my hy-power quit working before I could get the truck on the road. I'm looking for a working replacement.

Also, what is the status of yours - are your brakes up and working now?

Thanks in advance,
-Richard

[CarolinaHD]

Hey Richard,

I got everything buttoned up last week and took it for a quick spin but haven't had enough time to adjust anything yet. Spent this past week/weekend replacing headgaskets on my buddys Duramax and I'm pretty sore now haha.

I still have the brand new hypower booster and master cylinder but no backup pump if you're interested.
The earlier hypower unit worked like a traditional hydroboost setup, fluid flows from pump to booster, booster to gear box, 1 return from booster and 1 from gear box.

The brakes work too good now! They're very sensitive and lock up easy, gonna have to reconfigure some things and this is what I came up with..

The issues Im having are:

1. Hardly any pedal travel(less than 1") and stiff pedal. I can open a bleeder and press the pedal all the way down with no issues. So I know its working right and can have full stroke but I believe the issue is due to using a larger master cylinder bore. Factory single reservoir was 1.5" and the hydromax is 2". Since you are using front disc brakes I'm curious to see how well it works since you'll need more fluid volume.
There are several master cylinder options that fit the booster, including a 1.5" bore plastic reservoir master cylinder. I'm gonna order it this week and see if it gives me more pedal travel and I'll report back.

2. Rear brakes lock up quickly, gonna install an inline proportioning valve on the rear brake line to fix it.

[rkn463]

Originally Posted by CarolinaHD

Hey Richard,

Spent this past week/weekend replacing headgaskets on my buddys Duramax and I'm pretty sore now haha.

I hear that is a heck of a job, there is a good reason mechanics charge so much to do it! All of the crap under the hood of the newer trucks really makes you appreciate these old medium duty trucks.

Originally Posted by CarolinaHD

I still have the brand new hypower booster and master cylinder but no backup pump if you're interested.

Unfortunately, the auxiliary pump is all I need, the rest is all new (reman)

Originally Posted by CarolinaHD

The issues Im having are:

1. Hardly any pedal travel(less than 1") and stiff pedal. I can open a bleeder and press the pedal all the way down with no issues. So I know its working right and can have full stroke but I believe the issue is due to using a larger master cylinder bore. Factory single reservoir was 1.5" and the hydromax is 2".

I had the same experience with mine, check out post 223 http://67-72chevytrucks.com/vboard/s...&postcount=223 in my build. Keep in mind, my whole frame, drive train, brakes, etc. was donated from a 1989 C60 which I drove around a bunch prior to the change over and in that truck the brakes felt like a modern pickup. In post 223 you can see where after measuring the pivot to brake rod distance it was way shorter on the 68 effectively reducing the leverage that you apply to the master cylinder along with a shorter throw. Changing it would have required serious re-fabrication so I ultimately decided to go with it. One other variable that I introduced was a new (reman) master cylinder but it was supposed to be correct for the donor truck, they listed the same 1.75" bore size.

Originally Posted by CarolinaHD

Since you are using front disc brakes I'm curious to see how well it works since you'll need more fluid volume..

See above. Also, mine has 4 wheel disk brakes.

Cheers,
-Richard

[CarolinaHD]

Man I can't believe that little change would create such a great difference in pedal feel. I'm gonna get on my computer this evening and read your build thread on the big screen. This phone is killing my eyes. Thanks Richard! Couple questions,

Did your 89 have normal brake pedal travel before you swapped cabs? I just cant understand why the pedal doesn't travel as far as before. It should travel the same distance but require more force if everything else is the same and my thinking is correct.

Is your hydroboost on the firewall or under the floor?

[CarolinaHD]

As you said, the earlier units like yours had a 1.75" bore and later on they switched to 2". They also had a 1.5" option for the 5000lb front axle according to rock auto.

[Campbel45]

following.. this is very cool stuff you guys are doing. Keep up the good work!

[rkn463]

Originally Posted by CarolinaHD

Man I can't believe that little change would create such a great difference in pedal feel.

I'd run the math and figure out the pedal travel bit but my brain hurts today so I'm not going there!

Originally Posted by CarolinaHD

I'm gonna get on my computer this evening and read your build thread on the big screen. This phone is killing my eyes.

Good luck if you plan on reading the whole build, it went for a long time!

Originally Posted by CarolinaHD

Did your 89 have normal brake pedal travel before you swapped cabs? I just cant understand why the pedal doesn't travel as far as before. It should travel the same distance but require more force if everything else is the same and my thinking is correct.

Yes, the 89 felt like any modern pickup. The master cylinder swap may have contributed. Also, the clutch feels way different hooked to the 68 pedal set, it has similar geometry changes. When I first hooked it (the brakes) up I thought the hy-power unit wasn't working because of how hard the pedal was; but it actually has amazing stopping power, just no pedal feel. And if you kill the motor while you are rolling... well then you KNOW it was working! That is why I stopped working on the project, I consider it unsafe until I get a working auxiliary pump.

Originally Posted by CarolinaHD

Is your hydroboost on the firewall or under the floor?

It is on the firewall.

-Richard

[Campbel45]

CarolinaHD,

Did your new master cylinder come in? Able to fix your hard pedal and lockup issues?

[CarolinaHD]

Yes it did along with my inline proportioning valve. The rear brakes no longer lock up but the master cylinder did NOT fit. They had the wrong picture listed and it has 2 mounting holes to the booster instead of 4.
I found a website with various brake calculations and formulas and was able to determine that going from a 1.5" to 2" master cylinder bore reduces the pedal travel by ~80%. So 2" of pedal travel is reduced to less than 1/2", no good.

My next step is to swap in the extra master cylinder with a 1.75" bore. It came from the earlier hydromax style like Richard has so hopefully it will fit! I'm down in florida for the next 2 weeks so it will be awhile before I can update.

[CarolinaHD]

Brake pedal travel calculator

https://brakepower.com/help_abc_29_PTC_t.htm

Cheaper alternative for replacement motors, $112 with free shipping.

https://www.dbelectrical.com/product...SABEgLKJfD_BwE

[Campbel45]

CarolinaHD,

Thanks for the update. Great find on the calculator. It seems like with the 1.75 bore it will decrease pedal travel ~36%. Hopefully thats not too noticeable. If it is, maybe you can change your pedal ratio slightly?

Awhile back, I picked up a hy-power booster and master cylinder (I think 1.75 bore too). I havent used it because I was scared of trying to find expensive replacement parts for it. Another nice find on the back-up motor!

[Bigdually]

Originally Posted by CarolinaHD

Brake pedal travel calculator

https://brakepower.com/help_abc_29_PTC_t.htm

Cheaper alternative for replacement motors, $112 with free shipping.

https://www.dbelectrical.com/product...SABEgLKJfD_BwE

Is this motor for the auxillary unit as in Richard's truck, and also coincidentally in my own 89? My truck suffers the same crapped out auxiliary motor issue as his.

[CarolinaHD]

BigDually and Richard's truck utilize the Delco Hypower system which is the predecessor to the Bosch Hydromax. There were 2 different style motors and I have seen both styles on the Hypower and Hydromax systems. I think it all depends on the specific vehicle it was going in.

1 style has just the positive wire going to it and is grounded through the body to the chassis and the other has 2 wires, positive and ground.

They both have the same mounting holes, same inlet and outlet holes and same o ring seals. So in theory they should swap back and forth.

Somebody should buy that replacement pump and report back to us

[CarolinaHD]

Question for any of yall that has an inoperable motor;
Have you tried connecting power directly to it to see if the motor is good? Could be an issue in the electronic module, bad flow switch, or bad relay.

For my back up motor I plan to use a relay, momentary push switch, and mount it where the cigarette lighter goes. I cant find any junkyards willing to pull out that brake harness and module without cutting it up...

[rkn463]

Originally Posted by CarolinaHD

Question for any of yall that has an inoperable motor;
Have you tried connecting power directly to it to see if the motor is good? Could be an issue in the electronic module, bad flow switch, or bad relay.

Yes I have wired mine directly and still nothing. The factory wiring has a feature that it can sense if there is a broken wire or if the motor doesn't have continuity to ground - in that case the warning buzzer is sounded and mine does this. I think that either the wire broke internally or the brushes are bad or possibly an internal seal blew and the electric part is full of oil. For 112 dollars shown in your previous post I think I'm going to gamble on the newer pump working.

Originally Posted by CarolinaHD

For my back up motor I plan to use a relay, momentary push switch, and mount it where the cigarette lighter goes. I cant find any junkyards willing to pull out that brake harness and module without cutting it up...

If yours is a one wire setup, I can get you the factory wiring diagram (for the 89... it should work the same) and you could create your own harness where it just works. Keep in mind when you are driving and things go wrong, you might not have a free hand to work the switch!

-Richard

[rkn463]

Originally Posted by CarolinaHD

BigDually and Richard's truck utilize the Delco Hypower system which is the predecessor to the Bosch Hydromax. There were 2 different style motors and I have seen both styles on the Hypower and Hydromax systems. I think it all depends on the specific vehicle it was going in.

1 style has just the positive wire going to it and is grounded through the body to the chassis and the other has 2 wires, positive and ground.

They both have the same mounting holes, same inlet and outlet holes and same o ring seals. So in theory they should swap back and forth.

Somebody should buy that replacement pump and report back to us

Here is my take on using the Bosch aux pump on the Hypower booster. Since both systems run off of a power steering pump I would think the pressure that the aux pumps put out would be very similar. Flow, hopefully as well. The wiring part of it is a non issue as they need to come on under the exact same conditions so that should be fine. That leaves us with two potential problems:

1) bolt and port patterns. If this is different then it looks like to me a machine shop could make an adapter.

2) is the flow in and out of the pump oriented the same... If not then the adapter becomes far more complex. (I hope this makes sense)

-Richard

[Campbel45]

Hey CarolinaHD,

Able to try out the 1.75 Master Cylinder?

[CarolinaHD]

Nope, been crazy busy with work and life but I did get the rear proportioning valve installed. Took it on a 5 mile journey and did great but gonna need a valve for the front to get it just right.

The good news is there are no leaks anywhere on the truck and the new exhaust sounds great! Went and scouted a few potential parts trucks to make a dump bed but nothing worth buying yet. Having a hard time finding a pto for a sm420.

Anyways here's a picture. I need to make a build thread.

[Campbel45]

Great job, that exhaust looks clean!