Power-assist brake pedal "collapses" to manual level on 89 corolla.

On Tue, 18 Sep 2007 01:06:34 GMT, "Jeff Strickland" wrote: Thanks for your quick reply, Jeff. I really appreciate it.

Checking the brake fluid was the very first thing I did when the brakes started to go "soft". The brake fluid level is just a hair below the "full" line. So that is not the problem.

As I said above, the fluid is fine.

MY guess is that there is a vacuum leak somewhere; either in a line or in the diaphragm in the power assist unit. The next thing I will check is a possible leak in the vacuum line. Although I don't know if it would be loud enough for me to hear it. I suspect it would be if it were big enough to disable the power-assist unit. If the vacuum leak were THAT BIG, I assume that it would really be affecting the performance of the engine, which is running just fine. So I wouldn't expect that to be the problem.

Have you got any other ideas as to what it might be?

You are absolutely right on that. I have no intention of driving the car any further until the problem is solved.

Sincerely, BlackHawk

The pedal doesn't actually fall all the way to the floor; only about

3/4 of the way, at which point the peddle will not depress any farther without locking the brakes (with a LOT of additional foot pressure on it). The very marked increase in force required to stop the car makes me think that the power-assist unit has somehow been disabled.

That could very possibly be, as well. I can pull the wheels and have a look for leaks.

That makes sense too. If one of those "O" rings was letting go and allowing brake fluid to leak by, it could produce similar symptoms, I guess.

Do you think that a "code reader" could tell me what the problem is? Or are they more of a general nature that would only say "there is a low pressure reading in your brake system", which wouldn't tell me much more than I already know.

BlackHawk

A code reader will not diagnose the problem. If the brakes operate normally but need extra pressure on the brake pedal, then you have a vacuum leak or a bad vacuum booster. If the brake pedal goes to the floor with diminished braking effect, then there are several possibilities - a leaking wheel cylinder or caliper, a bleeder screw that was not closed properly, a bad master cylinder, or a leak in the braking system somewhere. The brake warning light makes me think that the fluid level is low or you have a problem with the master cylinder. If you do not have experience diagnosing and repairing brake systems, the problem you are experiencing may not be a good place to learn. I recommend having it diagnosed by a competent technician before driving it more.

The easiest way to check for a vacuum brake booster failure is: with the engine off, pump the brake pedal until it becomes firm & hard to push. While applying pressure to the brake pedal, start the engine. If the brake booster is working properly, the pedal will drop, due to the vacuum assist. Since the OP said: "The pedal just went straight through the power assist, down to manual brakes", I doubt the booster is faulty, but the brake master cylinder is shot and needs to be replaced, as others have recommended. Either the cupped seals are shot or the bore is pitted, causing the fluid to internally by-pass the seal. I'd recommend going with a new master cylinder, rather than a rebuilt. OEM of Aftermarket will do. Had too many rebuilds fail. Most brake master cylinder bodies are made of aluminum, with either a steel or aluminum piston shaft. The aluminum parts are anodized, which is fine until varnish builds up and the contaminants in the brake fluid eat away the coating. Then it's just a matter of time before (usually) the bore becomes pitted. There is also the odd chance that the cupped seal(s) split, but it doesn't matter, the end result is the same, brake master cylinder failure.

The brake warning light on and no fluid loss don't jive. The brake warning light comes on when the brake fluid level in the reservoir drops and the float-switch drops, turning on the light. Another possibility that I didn't mention before is badly worn brake linings, where the pedal has to travel a long ways.

After doing whatever repairs are needed, I would also flush all of the old fluid out and replace with fresh fluid.

Thanks for that, Ray. I will re-check the brake fluid, as I only checked the warning light some time after I had checked the fluid level, which was at the very beginning of my ride home. I noticed that the brake service light was on near the end, about 3 miles later, with perhaps 10 stops on the way. There was also a squealing noise at the right rear on left turns. I'll have a close look at the shoes and pads.

That right rear shoe may have a big problem.

Very good idea. BlackHawk

Thanks, Ray. I've done my own brake work for years, including rebuilding, and honing wheel cylinders. I WILL get the motors manual and Toyota shop manual from the library before I start on the master cylinder, if that is the problem. This is my first car with vacuum power-asssist, which is why I'm fixated on that component. I thought it might be a slam-dunk to diagnose; but if there is a BIG problem with that, I will take it to a shop. It's not going anywhere, except to a shop. BlackHawk

That makes perfect sense, Jeff. According to Ray O, though, I've erroneously reported the facts, which may in fact, be true. I'll have to have another look to make sure I haven't screwed up with that; as everyone's help here is predicated on the assumption that what I'm reporting are the TRUE facts. So, I will recheck everything tomorrow.

That simplifies things. Thanks. BlackHawk

The brake warning light also comes on when half the braking system fails - the 'spool switch' senses pressure in one half and no pressure in the other, shoves the spool to one side and trips the switch.

Pull the lead off the brake failure spool switch and see if the light goes out. If that's what's happening Park The Car NOW. Tow it for repairs. Probably has major problems with the seals inside the master cylinder, and it's time to install a rebuilt master cylinder.

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It sounds like you have some badly worn linings. The extra distance needed for the calipers or shoes to contact the braking surface will lower the fluid level. Hopefully, that is the problem and not a master cylinder. Good luck!

Thanks for that, P. I'll give that a try.

That sounds like one would have to do a VERY careful inspection of the bore and the piston before attempting to replace the rubber in the master cylinder. It sounds like ANY pitting or wear would preclude rebuilding. Thanks for the headsup, P.

BlackHawk

BIG SNIP

Thanks for that Jeff. BlackHawk

It is parked now, in any case.

Thanks,, Bruce. I haven't looked yet, but it sounds like there are two connectors on the master cylinder. One for the spool switch, and one for low brake fluid. Did I get that right? BlackHawk

The low brake fluid switch is a float attached to the reservoir lid.

The Spool Switch is a little aluminum block on the fender that might look like a simple manifold, but it has an electrical connector and wire going to it.

They will both be wired to the same warning light circuit, so the same wire splits and does both.

Follow the two brake fluid hard lines out from the master cylinder and the spool switch should be the next item in line. Not sure if they put the switch before or after the pump/controller in an ABS equipped car, I've never looked...

If the spool switch tripped, you lost pressure in one half of the brakes. The two ends of the spool are different diameters to stay balanced because the operating pressure is usually higher in the front discs. 1200/800 is normal, 1200/0 or 0/800 certainly is not.

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On newer cars the MC body castings are cheap enough that some of the "rebuilds" are in fact made with new castings, if they have to toss too many bad ones out with pitting issues. They'll still reuse the good pistons and internal parts.

On old cars where they can't get new castings for cheap (low volume demand) they bore out the master cylinder and insert a stainless steel sleeve. Then they drill for the fluid ports.

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Have manufactures gotten away from the pressure differential switch they used to put in dual systems? If you had a fluid leak in one system, the pressure difference would trip the switch and light the light.

No, I just had a senior moment, which Bruce was kind enough to remedy!