Dragging Brakes

[AJcolorado]

Whew.. yeah. The question is if I'm doing the master cylinder and have it off, does it make sense to do the booster at the same time?

[dlb]

the booster is easy to test (engine off, step on brake pedal until pressure builds up, then hold steady pressure on pedal - if the pedal sinks, the booster is bad) so i would just test it and if it seems good, leave it. i've never had one fail personally.

[AJcolorado]

Something I did notice when I took out the old p-valve was that one of the lines was dry, the other ones had evidence of break fluid in them, but one had little dried granules of something in it and no fluid.

Here's some words from a friend I have been corresponding with about this issue:

"I think the steps you just did may have pointed out the problem.

Before you changed the proportioning valve, the right rear and left front were sticking after running the car and pumping the brakes. Since changing out the p-valve caused both sides to stick evenly after run and pump, then you've potentially solved one half of the problem (the old pvalve was sticking).

Now, you've just got excess pressure in the entire system after running and pumping, and I assume no excess pressure if you don't run the car and pump the brakes up, right?

If that's the case, then I'm guessing the last half of the problem is that the brake booster push rod is out of adjustment (sticks out to far after pumping up the booster).

My proposed remedy, since we don't have the Toyota special tool for setting the pushrod adjustment accurately, would be remove the master cylinder, back the pushrod on the booster in a bit, then put the master on, bleed the master, bleed the brake system again, and see if that cures it. If it's not right, you'd have to repeat, backing up the push rod some more.

I do also have a pair of measurement calipers that I think has a depth gauge end. Using that on both the master cylinder and the booster, we might be able to figure out exactly (or at least close) where the pushrod should be adjusted to."

[rer233]

AJ- try pumping the brakes up till the wheels drag with the pedal released. Then loosen the brake lines at the master cyl. If the wheels now turn freely, it HAS to be a problem with the m/cyl or booster. Then re-tighten the lines, pump the pedal till the brakes drag again, and loosen the two nuts that hold the m/cyl to the booster. If the wheels now turn freely, there's a problem with the lash between the m/cyl piston and the pin that contacts it on the booster. The pin is threaded and therefore adjustable but for the life of me, i have no idea how it could get out of adjustment. You haven't done anything to the m/cyl or booster?? Isthe brake pedal coming completely up when you release it?

Good Luck- Keep us posted. Looks like your friend is pretty much on the seme page.

[AJcolorado]

Okay! When I pumped it up (with car running) and loosened the brake lines at the master cyl, they released. Then pumped it up again, and loosened the nuts between m/cyl and booster and the wheels really turned freely, all the drag was gone, even less drag than opening the brake lines or bleeders. So it seems that it must be the push rod.

There may be a slight sink in the pedal when pumped up but the breaks feel pretty squishy in general so I'll check for that after the next full bleed out.

The pedal does come all the way up, when released.

[Petros]

I find it hard to see a way how the brake push rod at the master could get out of adjustment. sounds more like the valve in the master is not releaseing the presssure, there are two, one for the front brakes and one for the rear. they are supposed to allow bleed back after you release the brake pedal, but leave some pressure in the lines. If they do not allow bleed back it will act like the brakes are not released. If the problem is isolated to front brakes only, that can not be the push rod or the booster, that would affect all of the brakes. the problem has to be in the master, specifically the part of the system that controls the front brakes.

I have seen very old brake fluid turn to brown chrystals, like brown sugar, it was in a car that has sat for 30 years without being driven. so it is possible that is the case, once all the lines flow freely with clean brake fluid, and you have part of the system locking up, it means something is not allowing the pressre to bleed back to the master. the only thing in the system that can cause that is (in order of most to least common) 1) bad flex hose (only isolated to that wheel brakes, not system wide), 2) the master cylinder internal valve (could affect front or back brake pair, or all 4), 3) brake propositioning valve (not that common, but they can go bad).

A bad booster and an out of adjustment push rod would affect all brakes. I can not see how a bad booster could make is stay "on", and the only way a pushrod would be out of adjustment is if you changed it when you had it out. The test for the bootser btw is to pump on the pedal several times with engine off, holding your foot down on the pedal, start the engine. If the pedal goes down, booster is good. the vacuum from the engine will assist your effort to hold the pedal down.

[AJcolorado]

After changing the proportioning valve the problem of locking and dragging was happening in both front wheels which seems to indicate the master cylinder over the pushrod which would effect all 4.

However, because rer233's suggestion of opening the nuts between master cyl and the booster released the break lock, I decided to try the pushrod adjustment and if it didn't work then replace the master cylinder.

So, I went ahead and took off the master cylinder and I see what everyone means about it being impossible for the pushrod to get out of adjustment by itself. However, went ahead and turned it in two threads, put it all back together, pumped up the breaks and... no more lockup. Took a careful test drive and everything seems okay.

Now...

I guess there may still be a problem with the master cylinder and adjusting the pushrod just makes it less obvious, for example if the master cylinder was getting stuck when pushed on hard by the booster and now it just doesn't get pushed in as far.

Also, before getting into all this I had taken it to two different mechanics and they could have adjusted things in the master cylinder...

...Or maybe just pulling off and bleeding everything clean could free up a clog in the master cylinder, or clearing out bad break fluid in the lines?

I will do some more careful test driving and see if any problems persist.

Thanks a million to everyone on the forum for helping, I know a hell of a lot more about breaks now!

[ARCHINSTL]

While it's a little late - did you read the section in the FSM on the m/c?
It's on BR-9, with the pushrod adjustment on page BR-15.
Tom M.