C4Ownr24

86 coupe.

Hi all - Some months ago I replaced my master cylinder and did a bleed at all the wheels - both manually and with a pressure bleeder. Ever since then, I've had pretty awful braking performance under heavy braking. When I went to get a smog test today, I was informed by the tester that I had no rear brakes - they ran the vehicle on the dyno but couldn't decelerate, go figure.

It's worth noting that my ABS light is on - I suspect I have a bad wheel speed sensor, but have just been ignoring it for the time being. I don't believe it's related to my lack of braking performance, but who knows.

I've also read that, when installing a new master cylinder, you have to manipulate the pressure differential valve (where the brake warning switch threads into) while bleeding the system.

Anyone want to take a swing at this? I'm quite certain there's no air in the system, but am also certain that I'm getting very very little rear brake pressure. Thanks in advance.

PatternDayTrader

I'm guessing you have air in the system. Hopefully someone will post the proper bleeding procedure, but I would bet your only problem is air.

C4Ownr24

That's possible, but I feel like I got a decent amount of fluid out of the rear calipers with the pressure bleeder. Anybody have any thoughts?

PatternDayTrader

Yeah even with a pressure bleeder it can trap itself up by the master cylinder. That's not the only place.

Tom400CFI

How firm is the pedal? How much travel do you have before the pedal starts to get firm/build pressure? Then how firm is it when pressure starts to build? Does the pedal build pressure smoothly, or is there sort of a "step" in the way that pressure rises? That is your indicator for air in the system.

C4Ownr24

Good firm pressure, no sponginess at all. I'm convinced there's not air in it (I should mention I'm a mechanic by trade, but have paid little attention to my own vehicles, over the years, ha.) I suppose a defective MC is also a possibility, but I feel like I may be missing something w/that pressure differential switch or the ABS system, which as I mentioned does not engage. IMO, that shouldn't matter, as these older GM systems are typically divided into primary/secondary systems and your pedal feel shouldn't change, even w/the system disabled.

PatternDayTrader

I sure hope someone posts the actual bleeding procedure. I don't know how long you have been a mechanic but I remember all kinds of weird stuff related to some early antilock systems. The first one that comes to mind is the bleeding the bpmv on early M/L vans that required loosening a set screw on both sides and then installing a special clip that held the end of a spool valve in a certain place. Obviously this isn't the same module as in the corvettes, but the point is there's a lot of bs crap that goes along with some early anti lock systems.

antfarmer2

I would clean the abs sensors and give them a good look. If they do not see the front wheels moving the back brakes will not work right.

Tom400CFI

^Are you sure about that? It would be pretty unsafe to design a system that caused the primary, hydraulic system to fail, when there was an ABS fault. AFAIK, all ABS systems in cars, when failed/faulted, operate as a basic, non-ABS hydraulic brake system.


How about this; can you get a partner to press the brake pedal, pressurizing the system, then you could go and crack fittings, starting at the MC rear circuit, then (if you have pressure/flow there), move on back through the system to the ABS module, then the rear calipers?

If the rear calipers really aren't working, at some point in the rear system, you'll not get flow/pressure when you crack the fitting.

antfarmer2

Try jacking up the back of your car and put it in gear and see how good the back brakes work.

Tom400CFI

That is a great suggestion. That should have been the first response in the thread.

Tom400CFI

There doesn't appear to be a lot of "bs crap" with the '86 bleeding procedure. Seems pretty straight forward.

Brake Bleeding.pdf

PatternDayTrader

Yeah that's pretty simple. I guess we will see what happens next ....

C4Ownr24

Pressure bled the system again yesterday, fed about half a liter of brake fluid through each wheel. No difference. While I had the car in the air, I applied the brakes in gear - in order to get any braking at all in the rear, I have to jam on the pedal with both feet, basically, so there is indeed a problem w/the rear brakes.

I did diagnose my ABS failure - the sensor in the right rear is damaged. I had a bad wheel hub a year or so back, which I eventually replaced, but I suspect that it broke the tip off the sensor before I got to it. Couldn't find the proper sensor anywhere online but I have a newer (89-96) sensor on the way, which I've read you can use, so long as you rotate it and adjust the position of the hold-down bolt accordingly.

I've read in other posts in the forums where people had my exact same complaint, then went out and did a few panic stops w/ABS application on gravel, dirt, etc, and ended up with a perfect pedal. So that's my next step - after that, I'll just chalk it up to a defective master cylinder and try another. Cheers.

JCrock

Did you have rear brakes before you changed the MC? Your symptoms are the same I had on my '86 when I got it, couldn't stop the rear wheels if they were off the ground without jamming on the pedal. New calipers fixed the problem.

Raysur

Rule out the simple things first. Was this master cylinder a rebuild or a brand new unit?

If new the dimensions can be different enough. You will need to adjust your booster's pushrod to meet up with your master cylinder. The brakes are biased to the front calipers so extending the push rod will engage the rear brakes. It is an easy job but needs to be within spec, you don't want the opposite problem of dragging brakes. You can unbolt and position the master out of the way so you don't need to open your brake lines.

MadDog87

Generally the proportioning valve will move to cut off fluid loss(as in brake bleeding). The valve needs to be centered again to restore the rear brakes. They can be touchy, but you can center the valve by re-bleeding the front calipers.

C4Ownr24

Got this fixed, wanted to share the results. Went ahead and diagnosed the ABS first and foremost - found the tip of the right rear ABS wheel speed sensor had broken off. I had a bad rear hub some time back so I'm sure it happened then. Replaced the sensor w/a one from a '90 (had to drill out a new hold-down bolt hole).

Gave the ABS a few applications on a gravel road, and what do you know, I've got rear brakes again, and a much much better pedal. Go figure.

Cheers

PM

antfarmer2

good job