Mushy Brakes

[Jaime]

My brakes are mushy. Most times when I use the brakes, the pedal goes down farther than it should before the brakes start engaging. If I pump the pedal once more, then the brakes engage normally.

I've bled the brakes having my wife pumping the pedal, and got a lot of air out of the line, but it took about a half hour, and I could tell that her patience was wearing thin. In any case after a day of driving, the problem returned.

I then used a motive brake bleeder, and pumped about a quart of fluid through the system, and the symptoms are still the same.

As a side note, I pulled the fuse in the engine bay for the ABS a couple of months ago because the ABS was activating frequently when not needed ( I reset all the sensors to no avail).

Any ideas as to what to do next to solve the problem?

Thanks

Jaime
97 D1

 

[Steve Rupp]

I would just open all the bleeders, remove the cap from the master cylinder, and just keep filling until you go through a bottle of fluid. Start closing from the pass rear on to the drivers front. Top off the reservoir. I'm not a big fan of the power bleeders. Let gravity do it's job.

 

[marc olivares]

could have air in the abs block. this happened to a buddy. hooked a test book to it and actuated the valving while bleeding and his mushy pedal went away.
or maybe your master is bypassing the fluid?

 

[Jaime]

could have air in the abs block. this happened to a buddy. hooked a test book to it and actuated the valving while bleeding and his mushy pedal went away.
or maybe your master is bypassing the fluid?

Thats something to look into since I did have ABS problems. Any other way of getting rid of air in the ABS block besides testbook?

I don't understand what you mean the master is bypassing the fluid... how do I check for that.

Rupps, I may try your bleeding technique next....just need to get more bleeder hose

 

[GregH]

I don't understand what you mean the master is bypassing the fluid... how do I check for that.

I believe Marc is referring to bad seals in the master cylinder. Only cure is replacement of master cylinder.

Out of curiosity, what brake fluid are your using? It's not Dot 5 silicone is it?

Also, check your bleeder screws for damage or leakage. One or more may allowing air into the system.

Are your brake lines in good shape? Are you still using originals?

 

[Jaime]

Greg,

I have the original lines with 128k miles. I've checked around the wheel area and see no signs of leaks either from hoses or bleeder screws.

I don't suspect a leak because the fluid level has not gone down in the reservoir. If I had some sort of leak, wouldn't the fluid level go down?

Do you know how to test whether the seals on the master Cylinder are bad?

And I've been using Valvoline brake fluid rated for both DOT 3 and 4 specs.


Thanks

Jaime

 

[scot]

Dont follow the "let gravity do its work" advice unless you really want to make it much, much worse.

Go get a bleeder kit, preferably with the bag setup, and bleed your brakes properly. This SHOULD get all air out of your system, including whats in the abs. Start from the caliper furthest away from the abs unit (not the master cylinder, since the abs in inbetween and adds length into the system) and start bleeting. This is done by putting an air tight connection between the caliper bleed nipple and a container of some sort using a properly sized clear hose. With someone in the truck they should press the brake pedal, while you slowly open the nipple until fluid starts to come out. If the fluid is coming out slowly, open it a slight bit more until it comes out fairly easily, but only open it to that point and no further. When the person in the truck gets to the bottom, close the nipple (just close, not he-man tight or anything) and then let the brake lever come up. This will suck fluid into the lines from the MC, and not allow anything in the line attached to the caliper to return. Repeat until the fluid is clear and no bubbles float up (might take longer than just getting it clear for the bubbles to stop as you are forcing them through the sytem). When you are done torque to manufacturer specs. Make sure the cup/bag or whatever you are using is ABOVE the brakes so the air bubbles float up easily, otherwise you run the risk of having bubbles return or be captures in the caliper. Every 5-6 presses refill the MC container (a third friend really helps here.) The first caliper will take vastly more work than the rest. Figure on this taking an hour or maybe an hour and a half. Valvolene dot4 "synthetic" is a good fluid and lasts pretty well. Stay AWAY from ATE super blue or other racing fluids. While they work extremely well, they are not designed to have an extended use life. The normal stuff you find at advance or whatever does not have this issue.

If that doesn't work, go get a set of stainless steel brake lines. These make a smaller difference than just getting the fluid free of air/water, but every bit helps. You will have to bleed again afterwords. Use a "flare" wrench to attach the nuts, or go get a ratcheting box end wrench. Makes life much, much easier.

When you are all said and done if you want that last 2% of improvement, somehow rig a board between the seat and the brake so it presses the pedal down. Leave it there for a day or so. The theory goes air bubbles in the MC will be able to "float up" to the MC while under pressure. Then a few pumps later the air bubbles come up and into the MC container and are released to atmosphere. I dont know if the theory is correct, but it does seem to work.


Scot

 

[Jaime]

Thanks for all the input. I seem to be on the right track.

Its not a brake problem per se, but rather the effect on the brakes of wobbly front wheels caused by worn wheel bearings.

I bled the brakes again, but no bubbles were seen. I drove around, and the problem was still there.

I went to take off the mud tires and put on road tires, and I noticed some back and forth play on the front wheels. So I opened the hubs and pulled the outer bearings an noticed a lot of wear. I repacked them and tightened down the hub nuts to eliminate the play. One side was so worn, that even though I bottomed out the hub nut and there was still a little play on the wheel.

Anyway, once I put the tires back on, and took the truck for a test drive, the brake pedal only bottomed out slightly. So I'm sure that once I put new bearings in, and eliminate the play completely, my brake pedal should be back to normal.

I guess that because the wheel/rotor was wobbly, the first press of the brake pedal went deep because the pistons were extending more than normal to center the rotor, and then the second press of the pedal was short because the pistons were already close to the rotor.

Just a guess on my part, but I can't think of any other possible reason given the results of my 'fix'