broke brakes (help!!!)

[BellCityDubber]

I seem to have a major problem that the shops around here just can't seem to fix.  I just did 2 major brake jobs on the car. once to do the rears in feb and once to replace the fronts. and my brakes are STILL GRABBING?!?!?

here's the brake history to date

Last march - Changed master cylinder and drivers front hard line
Dec - Changed front pads
Feb - Rebuilt rear drums - replaced master cylinder AGAIN
Mar - new flex lines and new front calipers installed on both sides in the front

the only things NOT changed on the braking system at this point are

2 flex lines at the rear axle
2 hard lines from the proportioning valve to each rear wheel cylinder
2 hard lines from the master cylinder to the rear proportioning valve
1 hard line from the master cylinder to the passenger side.

everything else has been changed and my fronts are STILL dragging slowing down the car. can ANYONE PLEASE solve this problem for me, I've taken the car to two shops, and I'm at wits end. Normally I would do my own work but I've been so busy recently due to work and family, working on my car has became a slim possibility.

I have reason to believe that the recently replaced master cylinder is already faulty. Considering I don't have a lot of money, I purchased a rebuilt unit. the first master cylinder was also a rebuilt unit and only lasted 10 months before leaking fluid into the booster

The bentley has this to say about brakes dragging
Brakes Drag, Bind, or overheat:

A. Brake Shoe Return Springs Weak (replaced in feb)
B. Brake Pad/Caliper or Wheel Cylinder / Shoe Binding (all items replaced)
C. Master Cylinder Faulty (last rebuilt unit only lasted 10 months)

I managed to pull the master cylinder off my parts car (87 gti) and it's pushrod is smaller than the replacement reman'd unit that I had replaced not too long ago.

I've heard that it may be that the pushrod in the booster may require adjustment.  I've also heard that they cannot be adjusted.  

here's a picture of my two master cylinders side by side for comparison.



As you can tell the one from the 87 gti (right) is about 1 cm shorter in overall length than the replacement reman'd unit.  

I would assume that the booster on the left will function until the brake fluid heats up.  At this point the brake fluid will not flow back to the resivoir via the replenishing port, because the pushrod from the booster will maintain slight pressure on the master cylinder tube, blocking the master cylinder from returning to it's rested state. (not blocking the replenishing port, allowing the fluid to be freed from the line)

anyone else have any ideas?

TIA

phil

[Darkness_is_spreading]

You prob have a certain amount of pressure already engaged on the master cylinder based on the difference in the shaft  length.  I would say try to find another master cylinder or find a whole unit master cylinder and diaphram and replace because your problem is pressure related.

[BellCityDubber]

found the problem, the one on the left is the WRONG master cyl for my application, the RIGHT ONE is the CORRECT one....

damn parts places giving me the wrong shiz...

d'oh

[BellCityDubber]

Alright.....

after driving around with the new master cylinder in, the brakes would bind, but it would take a little longer for them to do so.

Anyways, a vw friend of mine told me to (instead of pulling the master away from the booster) pull the vac line on the booster and see if the calipers retract.

Indeed they did, so I would assume this behavior meant that my booster is bad on the cab side and would have a leak causing automatic pressure preload on the MC Piston... correct?

[jtanguay]

hmmm i wonder if your problems are the cause of old corrosive brake fluid not being flushed???

[BellCityDubber]

in the past 3 months I've had my brake system flushed about 4 times.

I'm pretty sure it's the booster.... I've got this question up on a few boards and I failed to mention that in feb, when the reman master failed it leaked fluid into the booster.  it was probally leaking fluid into the booster since about nov.

about 4 months of brake fluid will most definatley kill the seals in the booster.

[jtanguay]

in the past 3 months I've had my brake system flushed about 4 times.

I'm pretty sure it's the booster.... I've got this question up on a few boards and I failed to mention that in feb, when the reman master failed it leaked fluid into the booster.  it was probally leaking fluid into the booster since about nov.

about 4 months of brake fluid will most definatley kill the seals in the booster.

yea that'll do it!  :shock:

hmm time for some preventative maintenance for my vehicle... i'll probably get one of those master cyl rebuild kits...

[hipifreq]

I have no idea how long brake fluid had been leaking into my car's booster before I bought it, but it was FULL when I overhauled the brakes. The booster still works just fine, but the vacuum pump seals blew over the winter. Luckily that's an easy fix.

I'd check out the VP if your booster has had fluid in it.

[BellCityDubber]

I know my booster still has vacuum... as the brakes will bind and I have to pull the vacuum tube from the pump to the booster to get the brakes to release.

So I know that the VP is making vac, I would assume that the brake fluid had deteriorated the seals between the front and rear chamber of the brake booster.

there must be some kind of minisucle hole where the rear chamber of the booster. (closest to the cabin) will drop in vac pressure, naturally making the front chamber (nearest the engine) have negative pressure. which would put pressure on the piston of the MC making my brakes bind...

this will happen after about 10-20 minutes of driving, after some time it will get increasingly harder to drive the car as there must be some binding going on with the calipers.

When I pull the tube, everything goes back to normal for 10-20 minutes again.