I have 4 wheel discs (stock iron calipers) and I have this weird problem that no matter what I do, when the engine is running I can slowly push the brake pedal to the floor with seemingly no pressure increase. When the engine is not running the pedal is rock solid.

I have so far replaced all the rubber brake hoses, brake pads, rear caliper seals, master cylinder, upgraded the prop valve with an adjustable wilwood. I bench bled the MC befor install and bled the whole system multiple times both with a vacuum bleeder and with the regular pump-hold-bleed method and it's still the same. I tried two other MC that I had laying around, same thing...
There are no leaks.

If I stomp on the pedal I can get the wheels to lock up, but with normally pughing the pedal in it never locks up. It get's to a certain level of braking and stays the same throughout the rest of the pedal travel.
The only thing I haven't replaced is the booster, but it holds the vacuum fine for days, and I really can't see how it could affect it.

I suppose there should be a point at which the pedal would stop even if the booster is working and I shouldn't be able to push the brake to the floor because the built up pressure would stop it. Also i have to push the pedal about half way in before any braking would happen. I have 4th gen pedals now, but it was the same with the factory ones too.

I need someone to point me to the right direction because it's driving me crazy.
Thanks in advance!

 

Did you remove the pressure switch from the proportioning valve and install a bleeder tool to keep the slide valve from moving during bleeding? If you didn't the slide valve can move during bleeding just as it does if you have a blown line; this will keep you from getting a good bleed.

 

I have replaced the stock prop valve with an adjustable wilwood unit (pn 260-11179). It doesn't require any special bleeding as far as I know.

 

I would eliminate variables to narrow down what is causing the issue.

First, plug the hole of the master cylinder feeding the rear brakes. This should be the front port of the master cylinder. I think your master cylinder has the standard, inverted flare outlets and will need a 9/16-18 inverted flare plug. Start the engine and see if your pedal improved. If you pedal improved, it is most likely the rear brakes giving you issue. If your pedal did not improve, it is most likely your master cylinder or the front calipers giving you issues.

Next, plug both holes of the master cylinder. The front port of the master cylinder that went to the rear brakes should be a 9/16-18 inverted flare. That should already be plugged. The rear outlet of the master cylinder is for the front brakes. The outlet on that should be 1/2-20 inverted flare. Get an 1/2-20 inverted flare plug and plug the rear outlet along with the front outlet. Start the engine and see if the pedal is rock hard. If your pedal got rock hard, then its your front brakes giving you issues. IF your pedal did not improve, then its your master cylinder.

I would guess its either your rear brakes or your master cylinder giving you issues. If you plug the front port of the master cylinder going to the rear brakes with a 9/16-18 inverted flare plug and your brakes improved, the issue is most likely your rear calipers. I am not sure if these can be adjusted by the parking brake or if there any adjustement at all with the rear calipers, but if you can adjust the caliper pistons out so that the pads are seated against the rotor, then this should cure the issue.

If you find that your master cylinder is giving you issues, you either have air in the master cylinder or a bad bypass valve that is built into the master cylinder. If air, the master cylinder will need to be bench bled off the car. To bench bleed, leave both plugs in the master cylinder's outlets to bleed it. Very, very slowly cycle the master cylinder piston with a NON sharp, NON pointy, blunt end rod. You will need to cycle the master cylinder piston very, very, slowly because there is a 100lbft bypass valve in this master cylinder. This master cylinder is a STEP BORE design that gives high volume AND high pressure for your front LOW drag calipers. If you cycle the master cylinder too fast, it may leave air in the bypass valve. After cycling the master cylinder piston very, very slowly it should start to get rock hard. After bleeding, there should only be about a 1/16" of piston travel. If you have excessive piston travel, you most likely have a bad master cylinder.

That being said, if you have a bad bypass valve in the master cylinder and you still have the original, 1983, from the factory LOW drag calipers, a failing bypass valve may cause symptoms of front caliper issues, but the bypass valve is not supplying enough volume of fluid to fill the front calipers, your pedal will have a long stroke and have low pedal with no pressure.

A hard stomp on the pedal might move enough volume to fill the low drag calipers and lock up the front brakes. A slow brake pedal stroke may not allow a malfunctioning bypass valve in the master cylinder to move the volume needed to fill a LOW drag caliper. If a LOW drag caliper cannot be filled up with fluid, you will have a soft, no pressure, low brake pedal.

When you did a hard stomp, did all four tires lock up?

I am almost certain it is not your front calipers, unless they were installed with the bleeder screw at the bottom instead of the top of the caliper. If the bleeder screw is installed at the bottom, it will hold air in the caliper causing a low pedal, but you most likely would not be able to lock up the brakes under any circumstance.

Sorry for being long, drawn out, and wordy. Let us know what you find.

 

Thanks David! I really appreciate your answer, and the more detailed the better.
I tried to use the search function, and although many has had the same problem, I’m yet to find one thread with a success story.

I think you are right. I also suspect the problem is with the rear calipers. Yesterday I adjusted the e-brake mechanism on the rear caliper, so that the rear brake pads are the closest possible to the rotor. On the driver side there is still a couple mm play, I couldn't get it any closer, but on the pass side it's tight against the rotor now.

After that, I started a two man bleed and found that the front brakes are fine, but there was still air in the rear lines somewhere. Especially on the driver side. I was simply unable to get rid of the small bubbles even after a quart of fluid.

I have already replaced the body line going to the rubber hose, and the rubber hose itself. I suspect there will be a pinhole or some other issue between the T of the rubber hose and the caliper, so next I'll replace the hard lines on the rear end. There is no visible trace of leakage, but these are old, rusty and ugly anyways.

The MC is brand new (ACDELCO 18M204), I did the bench bleeding procedure, and it took surprisingly long, however I'm almost 100% certain that there is no air left, because for the last 10 stokes I couldn't see any bubbles. But I will do it again, just to make sure.

Also I forgot to mention that the rear end is a borg warner unit with iron calipers. The car had 4 discs from factory, and I was thinking if the pistons are bigger in the 9 bolt calipers than on the 10 bolt factory ones, that could cause similar issue. So if someone has any info on that, it would be great.

 

I doubt the rear caliper piston size will be an issue. If there is any differences in piston size, it is very small and would not cause issues.

If you are pulling in air, this may be an issue, but if there is substantial space between the rear rotor and the rear pads, this will most likely be your issue. The rear part of the brake system will not build pressure until those pads touch the rotor. Up until it touches the rotor, it just pushing the volume of fluid needed to get the pads against the rotor.

There should be a slight drag on the rotor from the pad. This drag is not a real big issue because there is no pressure applied to the rotor. 1mm (about 0.040"), though small, is a substantial amount of area for the master cylinder to fill in a caliper. A step bore master cylinder only applies extra volume of fluid to the front calipers and not the rear. The front outlet of the master cylinder that supplied fluid to the rear brakes works just like a normal, strait bore master cylinder.

Look at it this way. The piston area of the master cylinder (24mm bore) is .701 square inches. Your rear caliper most likely has a piston diameter of 48mm. This equates to a piston area of 2.803 or roughly 4 times the piston area of the master cylinder.

So to move the piston of the caliper out 1mm (0.040"), the master cylinder would have to move 4 times as far to move the fluid volume needed to get the pad back up against the rotor. 4mm is a little less than an 1/8" of an inch (0.157" or 5/32"). A master cylinder has roughly 7/8" of piston travel, more or less. So this much excessive travel makes a big difference. The total amount of piston travel for this master cylinder should be around a half inch (0.50") give or take a 1/16" of an inch. Add in the extra piston stroke you need to move the pads against the rotor plus maybe some air in the system, then you'll be down at the bottom of your pedal stroke.

If you have a rubber line between the frame and the axle, safely clamp it shut using a clamp that will not hurt or damage the rubber line. Did you pedal improve? If so, your leak is after the clamp or the issue is with the drivers side caliper

If the pedal did improve, remove the clamp from the hose running from the frame to the axle and clamp off the rubber line from the axle to the caliper (if there is one). Did your pedal improve? If so, then we know its the caliper and potentially where the air is coming in.

I hope this helps somewhat.

Let us know how it turns out.

 

I had some time today, and worked on the brakes a bit.
First I tried clamping the rear brake hose, but I couldn't notice much change, so I bench bled the master cylinder again, and the whole system in the correct order. Also tightened up all the connections. I'm getting very good at it... I couldn't buy plugs for the MC yet, but I'll do that next time I get to the brake parts store, to test that theory.

I did a test run around the block about half a mile maybe, and since I adjusted the rear e-brakes to touch the rotors even when disengaged, it turned out to be too much preload, so it started to smoke very heavily. It smoked so much I thought it was on fire so I hosed it off. I know it's not the best idea to cool it down so rapidly, as it puts so much stress on the metal... It was so hot, the water evaporated instantly with a lot of white steam.
Next I decreased the preload, but since the e-brake lever goes on a hex bolt thingy, i can only loosen/tighten it in 60 degree increments. Did a test run again, no smoking, but the brakes just felt way worse than before. At this point out of curiosity I put back the preload again, just to make sure that indeed it was the preload that changed the brake feel and yes it was. I tested the brakes to see if they lock up, and they do. The handbrake is still completely useless.
All this preload changing was done on the pass side rear brakes, because on the driver side I couldn't adjust it, because turning the mechanism 60 degrees I cannot put back the lever, and with the current position there is still play in it.

So my conclusion is that you must be right and the pads should be as close to the rotors as possible but probably without much dragging or preload. I expected that the pistons inside the calipers would automatically adjust for the brake pad wear and therefore no shimming would be needed but apparently I'm wrong.
I looked up in the rockauto catalog, and they sell shim kits, so next I'm gonna order a few of those and try to adjust for a zero play with no preload. But it takes a week or two until parts arrive to Hungary, so in the meantime I'll replace the hard lines on the rear end.
I think we're going in the right direction. Thanks again Dave for the advice!

 

If you clamp off the rear line and take it for a safe drive (not too fast), is the pedal better?

This is, in my opinion, the best way to bench bleed a step bore master cylinder...

 

Alright, so now I have replaced the hard lines on the rear end. New fittings and all...
Now it's prettier than it was. The stuff rockauto sells as brake pad shims are basically just this stick on anti rattle plates. I stuck it on the rear pads anyways, and now they are a bit closer to the rotors, which helped with the effectiveness of the e-brake. It still won't stop the car, but it can hold it at least.

It was raining when I finished and I took it out for a test drive, so I can calibrate the brake bias on the wilwood prop valve. After some adjusting it locked up all 4 wheels at the same time, and I could regulate the braking force somewhat.
The pedal still sinks to the floor.

In the brake fluid reservoir I can see some black derbis, that I suspect is coming from the seals of the MC. I assume the MC I bought must have been sitting on a shelf for several years and the rubber seals dried out..
Now I've ordered a dorman rebuild kit for the MC. It comes with the rubber seals and even a new piston if the picture is right.
I hope it'll finally solve this problem.