Weird Brake Issue

[TheBrewery]

So I did a front disc conversion from POL I could never get the rears to bleed, it seemed like the old wheel cylinders had locked up... but I could drive it no problem and the fronts worked great. At this time I was planning on going with a hot cam so I would not have any vacuum so I added a vacuum pump for the booster.


So cut to this week and I had swapped out the rear cylinders:

The Good: Bled the brakes and I am getting good flow out of all four corners.

The Bad: The Pedal is not retuning to the top, if I turn the vacuum pump on I loose all pedal pressure and it goes to the floor so I would assume there is air in the lines, right? Bled it again, no air just a good bit of fluid. The issue persists.

Testing procedures: If I pump the brakes it will build pressure(again typical of Air in the line, but wait) with them fully pumped and the pedal very firm I turn on the vacuum pump and the second it kicks on all the pressure is gone, however if I leave the vacuum pump off then pedal pressure holds for over 10 hours which if there was air it would decompress and push the fluid back into the MC but it still holds so I have no clue what is going on now. Also when driving it (short distance and low speed) it will lock up the rears but seems like it does nothing on the front. I put it up on jack stands to make sure the fronts are clamping and they are with very little pedal movement.

Next I am going to check the clearance between the Booster rod and the MC to make sure it's adjusted properly. I am also going to use a reverse bleeder to make double sure there is no air in the lines(I really don't think there is, but I could be wrong). The only other thought is that maybe the Booster has failed and the vent in between the two chambers is not equalizing the vacuum and pulling the rod into the MC causing a gap between the pedal push rod and the back part of the booster. I am totally stumped, help please

[61K10]

how do they work with just eng vacuum? Have you tested that the vac/pump to make sure it's working correctly? can it run backwards? don't know if that even possible.

[TheBrewery]

Yes it can be setup backwards but we did check that and it is pulling correctly.

I don’t have any vac ports on the motor, so I can’t really say.

[SkinnyG]

Is the master for sure good? Sometimes when brake systems are possessed by the devil, it's a failed master.

[TheBrewery]

I just pulled everything apart, checked the Booster pin to MC clearance and there is .75mm of free play there so it should not be pushing past expansion/fill holes in the MC. I also used a reverse bleeder to push fluid from all four corners, so I know there is not a block in the line. I am totally lost, I think I will just buy a new Booster, MC and proportioning valve. Hell I may just bite the bullet and swap the rears to disc.

The whole system is brand new, POL disc brake swap, new rear wheel cylinders, all new hard lines, all new flex lines, all new fittings, on my second proportioning valve. I am almost to the point of scrapping all of it and starting over.

[Captainfab]

It sounds to me like the issue has to do with either the booster or the vacuum pump. How much vacuum is the pump producing? Do you have the vacuum pump plumbed directly into the booster or is it plumbed to a reservoir?

[TheBrewery]

Originally Posted by Captainfab

It sounds to me like the issue has to do with either the booster or the vacuum pump. How much vacuum is the pump producing? Do you have the vacuum pump plumbed directly into the booster or is it plumbed to a reservoir?

It's pulling between 15 and 20 inHg it has a pressure switch that kicks it on when it drops below 15, but it pulls about 20 to start. No reservoir, just direct to the booster. Just to note the pedal loses pressure the minute the vacuum pump kicks on, so it's not at full vacuum when it drops.


Note: sorry if this doesn't make sense I am drowning my sorrows in whisky

[SkinnyG]

I'm not sure I'm fully understanding what is exactly happening.

Are the back brakes applying by themselves as you drive? The MC piston is pushed too far into the master at rest. Either the rod between the pedal and the booster is improperly adjusted, or the one between the booster and the master, or both.

Is turning the vacuum on applying the brakes for you? The pedal rod is adjusted too far into the booster allowing atmospheric pressure to help apply your brakes.

But I don't think I fully understand what's happening.

[TheBrewery]

I have not tried to drive it without pedal pressure.

We did put it on jack stands and rotate the wheels and turn on the pump, it did not auto brake and still lost pressure. We also checked how far the pedal would need to travel to stop the wheel from moving with and without vacuum. With vacuum, you have to push the pedal about 2" farther to stop the wheels. That's what is really throwing me off, the vacuum system and the hydraulic system are separate the distance needed to move the caliper/cylinder should be the same regardless of booster assistance.

[61K10]

I would try a different booster first, before starting completely over. It is normal with foot on the brake pedal and when eng is started the pedal will move down some.

[Captainfab]

I agree on changing the booster first. POL should warranty it for you.

[TheBrewery]

I talked to POL last week, and they wanted to do some diagnostics. So at there request I blocked the lines at the wheels.

Starting with the rears the pedal dropped to just an inch above the floor with and without vacuum. Then I blocked the front (leaving the back blocked and the pedal stops about halfway with and without vacuum.

In previous conversations, this seems to point to MC being undersized for the setup. Does that sound right?

[dieseldude4bt]

If you have vacuum booster on the same pin as the manual brake master cylender travel will be two far with vac but will feel good with no vacuum.
Vacuum should mount an inch lower you will have to drill a new hole.

[TheBrewery]

Originally Posted by dieseldude4bt

If you have vacuum booster on the same pin as the manual brake master cylender travel will be two far with vac but will feel good with no vacuum.
Vacuum should mount an inch lower you will have to drill a new hole.

I'm using captianfabs booster bracket, and it locates it in the same area. Also I don't think it can go lower without hitting the shifter linkage.

[Captainfab]

You do NOT lower the booster/ master. You need to drill a new hole in the brake pedal down 1" from the original hole and move the booster pushrod to that lower hole

Originally Posted by TheBrewery

I'm using captianfabs booster bracket, and it locates it in the same area. Also I don't think it can go lower without hitting the shifter linkage.

[TheBrewery]

Originally Posted by Captainfab

You do NOT lower the booster/ master. You need to drill a new hole in the brake pedal down 1" from the original hole and move the booster pushrod to that lower hole

That makes sense, could that possibly cause the issues I am seeing?

[Captainfab]

Part of them.

Originally Posted by TheBrewery

That makes sense, could that possibly cause the issues I am seeing?

[dieseldude4bt]

In that case I would try a larger bore master cylinder.
It will move more fluid with less pedal travel.

[TheBrewery]

Replaced the MC and it's working now... went up to a 1.125" and added a RPV for the rears.