worf928

goodspeed928 and I are working on his '86 AT's brakes. Prior to the work, his car had a very soft pedal with lots of travel and very poor braking performance. He figured that new lines would help cure the problem.

Well, they didn't. The car has no brakes. I'm pretty sure that the master cylinder cups are dead and are allowing by-passing. But, I wanted a sanity check.

Here's the status:

The flexible lines have been replaced.

The system has been flushed - three times. The first time with a pressure bleeder and the last two times with the 'two-man' method. We didn't see any bubbles during the second and third bleed. I drove the car for a few miles after the second bleed and hence the discovery that the new lines had no noticeable effect. (I didn't think they would though.)

There are no visible leaks nor evidence of any leaks anywhere. Not from the line fittings, not from the bleed screws, not from the reservoir seals, not from the piston seals, and not from the master.

There is no evidence of brake fluid in the vacuum booster.

The pedal is rock hard with no vacuum assist.

But, once the motor is running and the brakes get vacuum assist you can push the pedal to the floor with moderate foot pressure.

Seems like the vacuum booster is working.

Master cylinder R&R?

DANdeMAN

Sound like master failure...but sometimes air gets stuck IN the master as well, specialy if you disconected the lines at the master, if not, its most likely the master.
A new one is way safer than replacing the cups and srings....

fbarnhill

If I remember correctly, that is just how the master cylinder failed on my old 83s. It was after doing the clutch, I couldn't get it to pump up. I replaced the slave under the car and it didn't help. I took it to the wrench and he discovered the master cylender had gone bad. The brakes had been a little spongy before that.

Good Luck

goodspeed928

Dave C. is going to get the "DR." Before is name.
His have had my car for 2 weeks (showing a lot's of LOVE and CARE for my car)
Thanks!

FlyingDog

Booster.

I called it in one of my first posts and the experts disagreed (on and off the forum). After bleeding it everyway possible (before and after changing the perfectly good master) they decided it could be the booster. I swapped the booster and the unbled (air definitely in the lines) brakes are far better than the previous perfectly bled brakes.

BTW, Watch ebay! I bought my new booster for a little over $110. I have seen several others go for close to that price.

worf928

Matt, there seems to be plenty of vacuum assist to the brakes. What failure mode of the booster would cause the symptoms I describe? While I have not ruled out that the booster is a possible culprit I would like to identify a possible failure mode consistent with the symptoms.

FlyingDog

I'm not sure how the booster could fail causing the symptoms I was having. Usually a booster failure causes a hard pedal. My failure seemed to be over vacuum assisting the pedal without assisting the brakes. When I have some time, I'll probably open up my booster and see if I can find what went wrong.

Phil Silano

If the master has failed wouldn't the pedal bleed down with or without vacuum?? Either way you are still putting pressure on the pistons and cups in the master.

worf928

There does seem to be a bit of by-pass with no vacuum. With no vacuum the pedal is firm but with moderate pressure the pedal will drop over a period of about 20 seconds. The master wasn't touched during the line change and the master was bled several times.

Phil Silano

Probably the master. When you pull it out check inside the brake booster for brake fluid. Mine had a leak out the back and the booster was full of fluid. Had to replace that unit also.

potsiewebber

i am having the exact same problem but havent figured it out yet.