Soft Brake Pedal

thehackmechanic

thehackmechanic

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I've been trying to sort out the brakes on this Tundra '73. It had terribly soft brakes and a seized rear caliper. I replaced the caliper. I then used my brand-new pressure bleeder to change the fluid, as the car had been sitting for 3-4 years. After bleeding, the brake pedal was better, but still soft, so I bled it again, and there was no change. I bled it a third time, again no change.

I then replaced all six rubber brake lines with new ones and bled it, and the pedal is still soft. I bled it again, no change. So it's had five bleedings. I'm sure I've run four quarts of brake fluid through it at this point.

Each time I bleed it, I'm doing master cylinder, then right rear, left rear, right front, left front, all with the pressure bleeder between 10 and 15psi. When bleeding the master, I crack open each of the five threaded lines for perhaps 30 seconds and let it seep onto a paper towel. On the front calipers, I'm bleeding upper nipple, then the nipple alone on its own side, then the nipple opposite that. I get good flow out of all of the nipples on all of the calipers, and let the fluid flow for a full minute.

If I pull the top off the reservoir so I can watch the level, it goes down between 1/8" and 1/4" when I stomp on the brake pedal. I've always associated a moving level with air in the line, as air is compressible but fluid is not. The level doesn't move when I hit the clutch pedal.

The pedal feels pretty firm if I stomp on it when the engine isn't running, but feels mushy when I run the engine.

I'm not seeing any brake fluid in the big vacuum boost hose.

I don't know how to bleed this thing any more, or any better. Am I missing something with bleeding the master? Or is the conventional wisdom that, at this point, the master cylinder is suspect?

--Rob
 
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I have never been able to bleed the system 100% on the first try. I have found that it takes 3-4 bleeding with some driving time between bleeds and I do not vent the lines at the master. I do use the pressure bleeder, but only with air. It seems using the brakes allows the small amount of air to finally clear. And I mean small - it takes only a couple of small bubbles left to make things feel soft. IIRC these last bubbles usually come out at the rear calipers.

It seems that if you get a hard resistance when the engine is off that the master is probably ok. Usually if the seals are failing you will be able to push through it - ie, feel it slipping.

Have you tried a bit of the old fashion 2 person bleeding method of push and hold, etc. Maybe getting the master moving a bit while bleeding would help?

Could the rear pressure balancing valve be frozen up?
 
I took my E46 Wagon (With 330I brakes and stainless hoses) in to a local shop because I could not get the pedal to feel as firm as the one on my E36 M3 despite repeated attempts with a pressure bleeder. I had rebuilt the calipers. The owner of the shop bled them and it gave no real improvement. Then he said back when he worked at Beach Auto (Toronto's most famous BMW dealer) they used to put the calipers on "tight" on Bavarias and 3.0 CS, because the seals were retracting the pistons too much when you let up the pedal. I was a skeptic, but said give it a try. My pedal was Rock hard after they did this and the brakes on my E46 are every bit as good as the M3's brakes.
 
It sounds like your brake bias valve is leaking. It is located at the back of the car up above the diff. Number 3 in this picture:

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It is not available new, but La Jolla Independent sells rebuilts on an exchange basis.
 
Andy, I can't favor that the bias valve is poor, if he is getting fluid from all bleeder ports on all calipers. This means that pressurized fluid is flowing to all calipers, theoretically carrying any air with it.

I'd take it out for a cautious drive, if/where possible, and get some heat in the brakes to see how it goes. Hack has another CS, so he ought to know what to expect.

Still, one wonders, that with the work that has gone into this car, what is it that's different from his other car.

Another thought is that one of the brake lines is leaking (though that ought to show up pretty quickly during a thorough visual inspection). This could allow the system to suck air back in when the pressure is released or pedal is not depressed. That said, inspection of all lines and hoses for leaks, and ensuring that all fittings are snugly made up, is in order.

Lotsa luck, Rob, and report back. No one yet has the answer despite several suggestions, and we'd like to know how it eventually works out. Without subscribing to the Roundel.
 
I vote for the buddy system - one presses, the other loosens then tightens the caliper nut at or just before the pedal travel maxes out. Repeat a few times at each caliper until no air and do the others. This is how I got my clutch slave to bleed and firm up. Then I'd suspect the cylinder.
 
Soft Pedal

I thought you had to bleed from the corner closest to the master, moving progressively towards the furthest.

Just changed some calipers on my E28, and the home pressure bleeder did an ok job, but I followed it up at the local brake shop and the pedal was a lot harder. Same for my E9.
 
Very common for the master cylinder seals to allow leakage and result in weak power brakes. Bleeding the brakes over and over won't help as there is no problem with the booster and no problem with air in the lines.
If you want to test for a small tear in the booster diaphragm to prove up the master cylinder failure, you can tee in a extra line after the check valve and apply the brakes while the engine is running. If your vacuum goes to zero while applying the brakes- your culprit is the booster.
 
Going through this now. Rebuilt calipers, installed new brake lines, rebuilt the pressure balance valve, and still have a soft brake pedal. All roads lead to the MC...
 
Honolulu said:
Andy, I can't favor that the bias valve is poor, if he is getting fluid from all bleeder ports on all calipers. This means that pressurized fluid is flowing to all calipers, theoretically carrying any air with it.
Click to expand...

His symptoms are EXACTLY the symptoms of a bad bias valve. You still get fluid to the bleeders and everything bleeds out like normal. It just leaks internally under pressure resulting in a spongy pedal. I chased my tail on this one for a couple of months before figuring this out. It's not just some WAG I threw out there.
 
Brake Bleeding

HB Chris, I rushed to check my Haynes and you're right - thank you.
 
force the calliper out a tiny bit, then try pushing it on as tight as you can. I did not watch them install my callipers, but the results were very impressive. Just pounded on my brakes moments ago.
 
Something is not correct in the manual regarding bleeding brake lines.

Hell, bleeding the brakes lines on our E9 according to manual must be something wrong.

First I bleed according to my own references how to do it. =BAD

Then according to Blue Book (about 3 times) =BAD

Then random opposite all known manuals and knowing = Suddenly PERFECT brakes! :)

Well, I can't be rich and famous writing a new version of the blue book how to bleed the E9-brakes, since I still don't know how to do it correctly :-(
 
Thanks

Thanks, everyone, for the input. I just corralled one of my home-for-the-holiday kids to pump the pedal while I bled it (for the sixth time) the old school way, with him on the pedal (and a Bentley repair manual beneath it, about the right thickness to prevent the pedal going town to far) and me on the bleed valves. It didn't make a discernible difference.

I need to think about this carefully. This is the Tundra E9 I have in the for sale section. Its value is capped by the rust. The pedal is slightly softer than I'd like, but it never goes to the floor. Between a master cylinder, a proportioning valve, and replacing the pitted rotors, I could easily drop five hundred on parts, and I'm not sure I'll move the value of the car by that much. If I'm feeling really ambitions, I suppose I could swap the proportioning valve, and then the master, from my red E9 to be able to diagnose the problem without layout out cash, but that's a lot of makework and brake fluid on the floor.

Everyone have a great holiday.
 
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