Have I mentioned that I hate working on brakes? Any ideas?

[ilomax]

Sorry, just realized I posted this in the engine form.:roll:   Hope I can still get some help.

So, I picked up a nice 81 Rabbit Truck and of course, the one think I hate to do is toast...The brakes.

I put in a new master cylinder, rear wheel cylinders, bled everything with my home made power bleeder (including the master cylinder) and everything is good... WRONG!

So here are the symptoms.

When the car is off, I have normal brakes.  Car stops (When I roll forward or backwards) like a champ and the brakes don't stick.  The pedal doesn't leak down, I can press it once wait a min. and press it again, works fine.  I press it down slowly and it feels normal.

When I turn the car on the brakes almost go to the floor and the pedal stays down there.  I have to pull it back up with my foot.

I have not check the vac. pump with the car on, but I did take it out and tear it down. I looked good, seals seemed to be in good condition, I would spin it and you could def. tell that it was sucking and blowing (ha ha ha).

I've had squishy brakes before, I've had air in the lines before, but never have I had normal brakes with the car off and almost zero brakes with the car on.

One thing that I can think of, is that I got the cheaper master cylinder and I was told that the push rod from the booster doesn't fit in all the way.  
So, I adjusted the push rod and it fit in almost exactly as it did with the original master cylinder.

If the push rod was out a little to far, would it cause the brake pedal to stick down?

Another thing that I thought, what if I didn't put the seal thats connected to the rod in the booster all the way in (Not making a seal).
Would that cause the pedal to stay down with the car on?

Please help, I would really like to fix and not have to touch anymore brake fluid.

THANK YOU ALL!

[jtanguay]

sounds like a bad booster to me... i would verify that the vacuum pump is working correctly too. (maybe one of those boost gauges with vacuum on it?)

some helpful topics:

http://vwdiesel.net/phpBB/viewtopic.php?t=5734&highlight=brake+booster+problem

http://vwdiesel.net/phpBB/viewtopic.php?t=7273&highlight=brake+booster+problem

[myke_w]

When I turn the car on the brakes almost go to the floor and the pedal stays down there. I have to pull it back up with my foot

I had this happen to a customer once and it turned out to be the booster, it was a new beetle.. that was the hardest booster I've ever done in my life..

[ilomax]

I had this happen to a customer once and it turned out to be the booster, it was a new beetle.. that was the hardest booster I've ever done in my life..

I've actually had that happen to me on a newer Jetta (MK4) and the ABS was causing the pedal slap (Up and down) at random times.

I compared the push rod in both of the master cylinders before I installed the new one and I thought I adjusted it to the correct spot or close to.

If I were to take an educated guess, it would be that the adjustment was out to far. Is it that sensitive of an adjustment?

How would you test the brake booster?

[myke_w]

disconnect the vacuum pump from the booster and see if it comes back up and acts like its off. I think this indicates a leak between the internal halves of the inside, like a hole in the diaphragm.

[ilomax]

disconnect the vacuum pump from the booster and see if it comes back up and acts like its off. I think this indicates a leak between the internal halves of the inside, like a hole in the diaphragm.

Duh...  That would make sense.  I get so frustrated with brakes and it's one of the easier things to do.  

Something about your hands hating you for days because you set the soak in brake fluid for hours.

So, that takes care of the booster, now how to tell if it's the booster or the MC?

I'm assuming that I'm taking off the hose, it should act like the car is off.

I'm starting to lean towards the MC not being adjusted correctly.

[myke_w]

I don't recall any master cylinder adjustment, unless someone has messed with it from the pedal side, maybe Libby can chime in here and help us with that.

[ilomax]

I don't recall any master cylinder adjustment, unless someone has messed with it from the pedal side, maybe Libby can chime in here and help us with that.

At the end of the push rod is an adjustment bolt.

If it is out of adjustment, how would you find the correct adjustment?

[ilomax]

The vacuum pump applies vacuum to both sides of the diaphragm normally.  When the pedal is pushed, a port between the two sides of the booster is closed and a port to atmosphere on one side is opened.  The vacuum is then only on one side of the diaphragm and so "sucks" the pedal down.  If the pedal freeplay (adjusted at the pushrod) is not adequate, the port between the two sides can be plugged and the atmosphere port open and the brakes always being activated results.  If the diaphragm was not sealing between the two sides of the booster, it would have no effect on the braking positive or negative.  I haven't found the adjustment specs in the Bentley.  I know I've seen it in one of the manuals, maybe Haynes?  The entire description of the procedure was also listed in a vwdieselparts thread that has since gone away with a server crash of theirs... :?

Andrew

Thank you sir, I believe you have made all of the pieces fall together and from the sound of it, the push rod isn't adjusted correctly.

You're a big help Andrew, thank you and thank you to everyone else the gave input.