CPP Brake Conversion

[Henney]

I've installed the CPP under cab booster/master/proportioning valve setup. I also have the residual valves installed and the system plumbed and bled. Been driving with it daily for a few months now and every now and again if I get on the brakes real hard they will hang up for 20-30 seconds before fully releasing. It doesn't do it all of the time and I can't figure out what's causing it. It also seems like it's only the fronts hanging up and more specifically the front left. Any ideas?

[yossarian19]

A hanging brake can be:
Stuck caliper (piston or slides)
Misadjusted booster rod
bad brake hose
and maybe some other stuff I'm not thinking of.

[dwcsr]

Unless the master is BELOW the caliper bleeder you do not use a residual valves. The purpose of a residual valve is keep fluid in the line and will do exactly what you describe.
Of the above things to check the booster rod is the most common problem we find with hanging brakes,

[dwcsr]

here is a diagram that explains when to use a residual valve

[OrrieG]

I had the same issue with my front discs during the first install, rod between pedal and mc was set to long keep pressure on the front discs which would heat up and lock. There should be a little play when you push the brake pedal before the rod contacts the mc or vacuum booster. I assume you are running a pedal return spring so you get full return. Adjust it at normal operating temps to allow for expansion in the rod.

[dwcsr]

Originally Posted by OrrieG

I had the same issue with my front discs during the first install, rod between pedal and mc was set to long keep pressure on the front discs which would heat up and lock. There should be a little play when you push the brake pedal before the rod contacts the mc or vacuum booster. I assume you are running a pedal return spring so you get full return. Adjust it at normal operating temps to allow for expansion in the rod.

There is supposed to be 0.020 clearance between the booster rod and the master. Rarely have I seen an aftermarket booster pin adjusted correctly out of the box.

[Henney]

Thanks Guys. I'll take a look at the clearance. We've adjusted it a couple times so I think its correct but...

[NW Task Force]

I have a 57 Chevy pickup with an aftermarket CPP style brake booster/master cylinder mounted on the frame (below the driver's side floor). I have tried many times to bleed the system of air and get the pedal firm before coming in contact with the floor. It seems no matter what I do, the pedal always reaches near the floorboard to stop the truck normally. Panic braking WILL NOT lock up the brakes. I find this unacceptable, especially if I have to emergency brake the pickup. I have heard stories that under cab style master cylinders are a pain to bleed the air because they are down low and air gets trapped. I am running residual valves for disc brakes (front & rear) also. Any tips, tricks, or things I should do to positively bleed the air out? Thanks

[dwcsr]

Originally Posted by NW Task Force

I have a 57 Chevy pickup with an aftermarket CPP style brake booster/master cylinder mounted on the frame (below the driver's side floor). I have tried many times to bleed the system of air and get the pedal firm before coming in contact with the floor. It seems no matter what I do, the pedal always reaches near the floorboard to stop the truck normally. Panic braking WILL NOT lock up the brakes. I find this unacceptable, especially if I have to emergency brake the pickup. I have heard stories that under cab style master cylinders are a pain to bleed the air because they are down low and air gets trapped. I am running residual valves for disc brakes (front & rear) also. Any tips, tricks, or things I should do to positively bleed the air out? Thanks

2 wheel disc, 4 wheel disc? size of master? how far down does the pedal go before it does brake, will it pump up once and brake better?

[NW Task Force]

Originally Posted by dwcsr

2 wheel disc, 4 wheel disc? size of master? how far down does the pedal go before it does brake, will it pump up once and brake better?

The truck is 4 wheel disc, about a 9" booster, and the pedal levers down several inches until you can feel resistance. The final 1" before the floor seems to actually engage the brakes.

[dwcsr]

Originally Posted by NW Task Force

The truck is 4 wheel disc, about a 9" booster, and the pedal levers down several inches until you can feel resistance. The final 1" before the floor seems to actually engage the brakes.

increase the master size to 1.125 or 1.25,

[dsraven]

check out summit racings brake tech. it talks about master cylinder size and brake pressures etc. larger bore can mean less pressure in the system if it is too large because the pedal effort becomes greater.

[dsraven]

if you can see into the master cylinder, have a buddy step on the brakes while you look into the cylinder (safety glasses). when the pedal is first applied there should be a little "disturbance" of the fluid in the reservoir. that is actually some backflow of fluid because the hole in the bottom of the reservoir is open to the bore of the cylinder and is forward of the piston at this point. this is what allows fluid to return to the reservoir after a brake application. as the piston is moved forward in the bore fluid travels up into the reservoir because it is easier than pushing the brake caliper pistons out. when it stops backflowing (when the piston travels past the bore supply hole), like very shortly after the pedal is depressed, the fluid is directed to the brakes and the further down the bore that the piston travels (more pedal travel) the more brake application. one thing to mention here is that a larger bore master cylinder is going to apply the brakes quicker (less pedal travel) because it dispalces more fluid per stroke, but the pedal effort will be more to get the same amount of pressure in the system once the brake pads have fully contacted the discs. a smaller bore cylinder will displace less fluid, so more pedal travel for the same amount of caliper piston movement, when the brakes are applied compared to a larger bore master, but once the brake pads contact the discs there can be more system pressure applied with the same pedal effort. when the brakes are released there will likely be another "disturbance" of the fluid at the very end of the pedal travel and that is the pressure in the system coming back into the reservoir. now, you may ask, if you step on the pedal and push X amount of fluid to the brakes, then release the pedal, all that should come back is the same X amount of fluid. well, that may be true but if the master cylinder was just a cylinder with no way for extra fluid to enter for top up, then as the brakes wear your pedal would get lower and lower to the floor because the brake calipers or cylinders have to be further out in their bores and so they displace more fluid. also, as you use the brakes when driving the fluid expands so the master cylinder has to have a way to get rid of the larger volume of fluid so the brakes don't stay partially applied. (this can happen when residual valves are used when they are not technically called for because they hold a couple of psi pressure in the system when there is already some pressure in the system from gravity).extra fluid is allowed back into the reservoir at the end of every brake application because the piston is allowed to come further back in the bore than the supply hole from the reservoir. that is whay pushrod length is critical. if the pushrod is too long the piston can't come back all the way and can cover that hole. if it is too short then you lose pedal travel taking up the slack inthe system before the pushrod contacts the piston in the bore.
make sense? hopefully not totally off base with the questions.
another thing I forgot to mention on the previous brake check blurb is to check the booster bracket and all linkage points for flexing when the brakes are applied. some mechanical advantage/pushrod travel can be lost that way plus, metal only bends so many times before it cracks.

[dsraven]

forgot to say " as you use the brakes the fluid expands from heat"
I know, boring guy, we all know this stuff.
a larger booster or a dual booster allows for a larger bore master cylinder. all good until your engine stalls while driving, then you have one good brake application before it is really hard to apply the brakes due to large bore master cylinder.
some guys will use a hydro boost in tight applications because of the larger master cylinder sizing allowed. they have a pressure reservoir in the hydroboost that also allows for a single brake application should the belt break or the engine stall.
i am sure somebody on here has some pics of a hydroboost install in one of these trucks.

[NW Task Force]

I went out into the shop tonight and decided to investigate some more on the master cylinder. After wiping it clean yesterday, two areas had pooled up with drips over night. One was directly below where the master cylinder mounts to the booster and the other was directly below on the center section of the MC. I ended up taking off the MC and noticed fluid residue at the bottom of the rear piston bore. I also noticed fluid was weeping around the edge of the MC cover lid. The entire MC/booster unit has only 3 yrs of use on it (occasional car shows), that's it. It's hard for me to believe the MC is leaking from the piston. The bore of the piston is indeed the standard 1". The casting on the MC is 29969, which indicates a standard GM master cylinder used on corvettes. Apparently this number is generic because it does not specify disc/drum, disc/disc online. I attached some pictures for clarification on leaks areas.

Should I buy a new MC with the 1-1/8" piston and re-seal all the brake lines attached to it?

Note: No fluid/residue was found inside the vacuum line to the booster and the pin area of the booster to MC.

[dsraven]

could be just the lid leaking. does it look like there is a pin hole in the casting?
if it was me I would probably take it apart and check what you have for guts inside and also see if there are any pit marks in the bore. but if you are doing that then you might as well put a kit in it, and if you are doing that then you might as well just get a rebuilt that is specific to a vehicle with disc/disc brakes. that way you know the master is correct at least. you could also get a used one with the large bore from the wrecker just to try it but that is sort of a waste unless you keep it installed after the trial, in which case it isn't a waste. on and on and round and round.
brake fluid likes to trap and hold onto any condensation etc, so the existing cyl may have some pitting even though it has seen little use. one of the downfalls of having the thing under the floor close to the road is that it also is out of sight out of mind. it kinda looks like it has seen some moisture. another thing to check for is fluid inside the booster from a possible leak at the bore of the master. I usually like to use a plastic straw or cable tie to check because they are not likely to cause damage to the diaphragm inside the booster. the pics kinda look like the bore was leaking but then there is also fluid under the cap gasket area so that may be the problem and it just ran down from gravity. same as the leak partway down the casting.
as far as going with a larger bore size, it is hard for us to know because we don't know what kind of pedal effort was required for an emergency stop with the current cylinder set up.
did you also have a chance to check all the linkages etc and the pushrod clearance?

[dwcsr]

Originally Posted by NW Task Force

I went out into the shop tonight and decided to investigate some more on the master cylinder. After wiping it clean yesterday, two areas had pooled up with drips over night. One was directly below where the master cylinder mounts to the booster and the other was directly below on the center section of the MC. I ended up taking off the MC and noticed fluid residue at the bottom of the rear piston bore. I also noticed fluid was weeping around the edge of the MC cover lid. The entire MC/booster unit has only 3 yrs of use on it (occasional car shows), that's it. It's hard for me to believe the MC is leaking from the piston. The bore of the piston is indeed the standard 1". The casting on the MC is 29969, which indicates a standard GM master cylinder used on corvettes. Apparently this number is generic because it does not specify disc/drum, disc/disc online. I attached some pictures for clarification on leaks areas.

Should I buy a new MC with the 1-1/8" piston and re-seal all the brake lines attached to it?

Note: No fluid/residue was found inside the vacuum line to the booster and the pin area of the booster to MC.

I would go with the 1.125" bore I think you'll be happier with the pedal. It will feel slightly harder to press but not significantly. M39052 over on rockauto.com is about $28.

[dsraven]

yup, i'm with DWCSR, stick a new one in and, for that price, if it doesn't work out you are only out a few bucks and some time. probably be fine though. make sure you bleed it really well and adjust the linkages.

[NW Task Force]

Yes, I agree! I am looking at buying a new master cylinder with 1-1/8" piston and the matching disc/disc proportion valve. This one to be exact:

http://www.ebay.com/itm/Disc-Disc-Ma...9S-6~u&vxp=mtr

What is the difference in advertising for 2-port vs 4-port? I assume 4 port has 2 outlets on each side of the master cylinder body?

[yossarian19]

FWIW, a 1 1/8" master cylinder is more than enough cylinder diameter for running big twin-piston front calipers + rear disc. I know this from having put a 1 1/8" master on a Jeep I had with the brakes I mention... pedal felt great but I did lose some brake performance, I think, vs the 1" master that was in there. Set up from the factory, my daily driver (with the same brakes as the old hot-rod jeep) uses a 1" bore.