I'm having an engagement issue with my brake Pedal. Here's everything as succintly as possible. The Master Cylinder died recently and has been replaced

Car: 1988 GTU (four piston calipers)

The following parts are recently replaced:

All 4 rotors

All 4 sets of pads

All 4 calipers

Master cylinder


I put in the Master Cylinder today. Thoroughly bench bled, all four calipers were bled as well.

Here's what the pedal does:

Engages slightly low, but very hard (grabbing).

Additionally: I've been fooling with the Master Cylinder pushrod to get a nice engagement point. Although I can get the pedal to engage high, it's very grabby.

 

This one is a real headscratcher. I moved the pushrod up and it engaged higher, but grabbed hard. After a few laps around the block, one of the calipers started to drag until the car stopped and would not budge in gear. After I loosened the pushrod, the brakes returned to normal.

The basic problem is that the pedal engages half way down. The engagement is fine when it occurs, but the pedal doesn't really grab until it's half way to the floor.

Here are my thoughts:

1. Not booster related- If it were the booster the pedal would become harder and a booster in general has nothing to do with low engagement.

2. Not Proportioning Valve related- A proportioning valve problem wouldn't cause a lower pedal engagement point since the fluid would still have to engage one of the brakes and thus offer resistance.

3. Not Master Cylinder related- The master cylinder was replaced today. All the ports were bench bled until even the little tiny bubbles were gone.

4. Not caliper related- There's no fluid loss. A frozen piston would not cause the pedal engagement point to be low but remain the same.

That leaves... well just about nothing. I've been thinking this over for quite some time. Thinking back on things. I've realized one thing: I'm not positive I replaced the rear rotors when I did the pads. I thought I did, but I'm not positive. I know I did the front rotors and pads.

Perhaps this problem is caused by rotors that are overworn?

Any other insights out there?

I'm going to see if I can determine if the rear rotors are still good. I wish I had a micrometer!

 

Piranha seems to think that even low rotors wouldn't cause this problem. Low rotors are adjusted for by the MC when you pump the car up. He says Jimmyv13 ran his rotors down to the vents; by that logic, he'd have to push his pedal to the floor to stop. My rotors certainly aren't to the vents yet.

Only other guess is air in the lines. *shakes head* I have composed a poem to help facilitate this conversation I'm having with myself.

Stupid air
Are you there?
Life is unfair

My brakes have broke
And the forum answer is quiet
Making me wonder
Like the air
Is it there?

It is tough to find an invisible physician
Yet tougher
To be a physician of the invisible

 

Ahh, reverting to poetry did the trick...

Nope, worn rotors wouldn't cause it...

I think YOU playing with the booster rod prolly caused most of your problems, because then the MC piston is traveling at different throws than it's supposed to (which could explain your locked-up caliper- you blocked off a return port in the MC).

COULD be a booster problem too...

I'd hate to think it was an installation or bleeding problem (if the peddle's hard, the bleeding's prolly good)...

 

How could the problem be booster related? What would cause this? A vac leak? Wouldn't that make the pedal hard? What does the booster have to do with the engagement height?

 

Got FSM?

There's a booster check procedure in there somewhere.

 

Sadly, this seems to be kind of normal. You can't adjust the pedal up high with the pushrod adjuster or you get the whole brake dragging problem.

I think it might just be the way the brake system works, with no real solution.

-=Russ=-

 

not to hijack, but id like to raise my pedal engagement an inch or so, when im ON the brakes, its low enough that i hit the gas too, unless i have my feet in the EXACT right spots... any tips?

 

Yes, I've got the FSM. But I don't understand how a bad booster could cause a low pedal engagement?

 

I drove his car, and it's peculular. The pedal starts out softer than normal, and grabs HARD. Normally to get it to stop the way his car does I have to hit the brakes pretty hard. After you push through the inch or so of softness, it feels like a wall. Almost like when I drove around without my brake booster not hooked up a while ago. The thing is, you can push through the softness, the brakes grab pretty hard, but you cannot lock the wheels up.

 

Neither do I offhand, but booster problems are funny things...

All right, for now throw out the booster hypothesis. Figure out what you did wrong with the rod adjustment...

 

I wasn't arguing against it. His brakes are so weird I wonder if that is the case. I'm no expert in brakes so I can't really say either way.

 

Well I'm going to retest the booster today and possibly have the car professionally bled. I've bled it so many times that I'm getting pretty tired of it.

 

Could it be a bad MC? It sounds like maybe the plunger is sticking untill there's enough pressure to push past it.

 

I was thinking about this as well. The symptoms do fit that description. I hope it is not the MC because it was just replaced.

 

Alright, here's an update on what I've done so far today.

I rechecked the booster check valve in the line that goes to the engine. It passes (flows only to the engine and not to the booster.)

I ran the FSM Booster checks. The first two checks pass easily. The third check is close. I don't think the pedal is dropping, but it could be dropping just slightly. It's hard to tell whether my pressure on the pedal is even.

I also adjust the pushrod so that it is close to zero space between the rod and the engagement of the piston. I figured out that if I move the pedal very slowly I can feel the moment the pushrod contacts the piston. There are two distinct pressure variations:

1. The free play; which is very small ( a few mm.)

2. The rod contacts the piston; shortly after the freeplay is eliminated though not immediately.


I have two goals now:

1. Search for Vacuum leaks.

2. Rebleed.

I was expecting today to have no answers and no questions. Instead I just have lots of questions. I feel like if I just keep tinkering with the thing something will happen that sheds light on the situation.

 

I was unable to find a vacuum leak. Time to bleed again. After that, I'll swap booster.

 

Okay. 3/4 way done with second bleed run. No improvement.

The pedal engages strongly but only halfway down the stroke. The first half of the stroke elicits only the reciprocity of the pedal spring.

 

Alright. 4/4 done with bleeding. Still same results.

My guess here is a partial booster failure.

I'm going to double check all the pedal specs and then start figuring out how to acquire a booster.

 

So a few days later, I figure out my problem. Faulty brake booster. Here's the lesson:

A bad brake booster can create a soft pedal and not just a hard pedal. If you're adjusting the pushrod more than six threads or so out, the booster is a definite possibility.

The symptoms I had were:

Soft pedal for half of the travel
Hard pedal for the second half
Harsh jarring engagement
Inability to lock up the brakes

 

closure! i too just dealt with brake booster failure, but mine was more obvious and easy to locate. however, the first booster i got, the master cylinder didnt fit over , so now im awaiting a new one. it had better solve all my problems or ill cry.

 

Interesting... I'll have to replace mine and see if it fixes any of the goofy issues I've been having.

-=Russ=-

 

Good information. You can't always trust a new part I guess.

 

I've been suspecting the Booster in my situation as well. After a lapping day last season the brakes became soft. Since I've replaced the front 4 piston calipers, switched to Steel Braided soft lines, replaced the master cyclinder, replaced the rotors and pads on all four corners. The car bites hard, but there is still too much initially pedal play. I'll purchase a new a booster shortly and see what happens.

 

BACK FROM THE DEAD!!!!


Any news?