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Discussion Starter · #1 ·
'88 Shelby Z w/ 4 wheel disc brakes.

I have tried searching through the topics and haven't found this problem; maybe I missed it? If so, apologies- but I did try.

I have replaced all four rotors, calipers, pads, master cylinder, and hoses (w/ stainless steel). I have bled and bled and gotten all bubbles out. We used the old fashioned up/down two-man bleeding method and a pneumatic bleeder. In both cases, we had the engine off and made sure to keep the brake reservoir full. We got a great feeling pedal. Once you turn the engine on, however, you put the pedal on the floor; or as my friend said during his test drive "I think I just kicked your radiator".

We bled passenger rear, driver rear, passenger front, then driver front. The MC was bled in the car using the pedal but with the bleeder hoses connected as you would with a bench bleed. (I don't have a vice)

I am off to the garage to try to keep bleeding, this time with the engine running - only because I am out of ideas. Any thoughts?

Thanks in advance to all!
 

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Welcome To TD!!!
I have run into this before and it sounds like you have a faulty master cylinder.
The only difference between the engine running and off is power assist.

Replace the MC, bench bleed the MC and then bleed the wheels...
R/R/-L/F L/R-/R/F
The system is a split diagonal, one port of the MC operates L/F and R/R and the other R/F and L/R and they need to be bled that way.
 

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Discussion Starter · #3 ·
Thanks for the feedback NAJ. I didn't know about the bleeding order. I did it passenger rear, driver rear, passenger front, the driver front. I guess I thought I should go farthest from m/c to nearest.

Sunday I bled them AGAIN, and more air came out; then I took some laps around the neighborhood and they got better. Still not there, but better. I guess it may take a few more sessions to get it all out.

It could be the m/c but it is brand new (not reman), so unless it is a manufacturing defect that shouldn't be the problem.

Next time I bleed, I'll do it in the order you state. Hopefully, that will help.

Thanks again!!
 

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When you rebleed the system, recheck your pads. Make sure they're in the right place. I've had a soft pedal before when I had a pad out of position.
 

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You don't have the calipers switched left to right, do you? It is possible in the front, but I am unsure about the rears. The only difference in the fronts from left to right is the location of the bleeder screw. It has to be up high on the casting.

Usually this mistake results in very little or no brakes, however.
 

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Discussion Starter · #6 ·
I think I might have it solved.

Basically, since I replaced everything, I guess I just underestimated how long it would take to completely bleed the system. I bled it until no bubbles came out, and drove it to find soft brakes. I bled them again, drove it, soft brakes but better. And repeat. I did this several times, and they seem to be back to normal.

Thanks to everyone for their help and input. This is a great forum.
 

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I have ran into the same problem lately, we have replaced all 4 calipers, pads and shoes and rotors and the MS and PBB..
My son bench bleed the MS and even swapped out the MS for 3 other units and still the thing does not seem right, you can get a hard pedal until you turn the car on..down to the floor it goes...

If anybody lives near the Cincinnati/Dayton area please contact me direct , at this point I need to get this car back on the road and any help would be awesome.

[email protected]
 

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I have ran into the same problem lately, we have replaced all 4 calipers, pads and shoes and rotors and the MS and PBB..
My son bench bleed the MS and even swapped out the MS for 3 other units and still the thing does not seem right, you can get a hard pedal until you turn the car on..down to the floor it goes...

If anybody lives near the Cincinnati/Dayton area please contact me direct , at this point I need to get this car back on the road and any help would be awesome.

[email protected]
Are you calipers installed right side up?
 

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Was this problem present before replacing all of the brake parts?
 

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95.0% of the time a brake pedal drops to the floor it is due to a hydraulic issue.
1)Faulty Master Cylinder
2)A Leak
3)Air In The System
The other 5% would be wrong parts or parts improperly installed.

To Check the Power Booster...
1)Pump pedal until it is hard
2)While holding foot on brake pedal start the car, the pedal should drop.
3)Shut the car off and pump the brake pedal.
4)You should get 2-3 assisted brake pedal pumps before the pedal gets hard again.

1)When bleeding always start with the MC.
It is best to use a MC bleeding kit then install on the car, have a helper hold their foot on the pedal then loosen each line one at a time until fluid squirts out the close the line.
Repeat this until no air is present.
2)Gravity bleed the system
Jack up the front of the car, open the R/R bleeder and leave it open until a steady stream of fluid is coming from the bleeder screw, then close the bleeder.
Move to the L/R wheel and repeat.
Lower the car and jack up the rear and repeat the process on the R/F and L/F wheels.
3)Bleed the system R/R, L/F, L/R, R/F with a helper to work the brake pedal.
Have your helper pump the pedal and hold, you open the bleeder until fluid squirts out then close the bleeder.
You should not allow the pedal to go to the floor when bleeding.

You can also try a MityVac to bleed the brakes or hose on the bleeder screw inserted into a clean jar/bottle filled with brake fluid and pump the pedal slowly until no air bubbles are present.

NOTE...
Do Not allow the fluid level in the MC to drop below 50% when bleeding.
 

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Reviving an old thread. I bought a CSX that had no brake pedal effort. When I went to bleed the brakes I only got fluid out of one of the front calipers. .I replaced all the bleeders, all four rubber brake lines, new MC (bench bled it). Disconnected the RR hard line when I did the new rubber lines and nothing came out. Verified fluid was coming out of the MC to feed the rear brakes proportioning valve. Pulled the proportioning valve and found a bad seal on one of the internal pistons/valves. Repaired that seal and now have fluid from all four corners.

Used darn near a gallon of brake fluid bleeding/flushing the system and still no help.

Hard pedal with engine off, but as soon as you start the engine, the brake pedal will go to the floor. I just verified all brake pads have good meat so not just excessive pedal travel.

I just don't get why rock hard pedal with engine off, but goes to the floor with engine running. Oh, I have verified LF and LR caliper are moving when pedal is pressed, but not very much. Zero leaks anywhere in the system.

Any yes all calipers have the bleeders on the top of the caliper (proper installation).

Any suggestion would be appreciate.
 

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Keep in mind that the engine running only creates vacuum assist... Simply more pedal pressure.

It would be interesting to know if braking ability with no vacuum is the same as with vacuum.

Also, pinching off rubber hoses to isolate the problem is helpful. You can even plug off the master cylinder outputs to ensure that the problem isn't there.

I have seen brake calipers and wheel cylinders suck air in but not leak anything out. Do not ask me how that is possible, but it is.
 

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The pedal will be hard with the engine off, with the engine running you are using vacuum assisted braking, when the engine shuts off you will still have 2-3 assisted pedal pumps before the stored vacuum in the booster is depleted.
Normal operation of the booster...
Pump pedal 2-3 times with the engine off, the pedal will be high and hard.
While still holding your foot on the pedal start the engine, the pedal should drop slightly and have a softer feel.
Shut the engine off and pump the pedal, you should have 2-3 assisted pedal pumps.

From my past experience if your pedal goes to the floor and you have bled and bled and bled you have a faulty master cylinder, new does not always = good.

Raise the vehicle in the air, start the engine and apply the brake, are all 4 wheels locked?
 

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I get the whole vacuum assist, but the pedal should not travel to the floor with the engine running.

I.am.going.to try and put the original MC back in and see what happens.
 

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I am assuming "MC" means "Master Cylinder."

"New" and "Good" doesn't go hand in hand as often as people want to believe.

Plug off one outlet of the master cylinder at a time and see what happens. It might be easier. It irritates me when people won't diagnose and just swap parts around.
 

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Well I appreciate your input, but I guess your going to stay irritated. When you troubleshoot a system with no results, eventually you must change parts, especially on a car that Sat 9 years.
And yes I am fully aware new does not always mean good. Still does not answer why a hard pedal with engine off and soft pedal engine on. A faulty MC does not only fail with engine on. A bypassing MC would do it all the time and not just when boosted.

Or would you care to tell us all how that can be?
 

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Did you raise the vehicle, apply the brake and see if all of or any of the wheels locked?
 
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