Pedal to floor when engine running

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Heysq86

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So I have installed new front calipers, front hoses, new PV and new MC and then have gravity bled all of the brakes (more than once) and have a firm pedal. When I start the engine, the pedal goes straight to the floor. What is up?? I did bench bleed the MC and have verified that the booster check valve is good. This is a 1986 C10 with a Gen III swap. And the rear brakes have been adjusted correctly.

Has anyone experienced this and found a solution? NEED HELP !! Thanks
 

legopnuematic

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You might need to vacuum bleed them. On my 76 I had a rock hard pedal when not running and when running it was worse than spongecake. Tried to conventionally bleed the rears and made no difference. Hooked my vacuum pump (old refrigerator compressor) with a catch can and cracked the bleeder and got lots of bubbles. Did both rears and the pedal became firm with engine running.
 

PrairieDrifter

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It can also be helpful to bleed brakes while it’s running. Sounds like you’ve still got air in the system
 

chengny

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So I have installed new front calipers, front hoses, new PV and new MC and then have gravity bled all of the brakes (more than once)

The big questions are;

Did you do all that work to solve an existing low pedal condition?

Or, was the pedal travel normal (and the work was just done as preventive maintenance) up till that point? But then, only after the renewals were completed, it became excessive?

Reason I ask is, maybe the issue is due to a defective part - or one meant for a different application. For example, maybe the MC is bolted to a vacuum booster it isn't compatible with.
 

Heysq86

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Chengny. thanks for the response. Did a GEN III swap in my truck so this is new condition basically. I drove the truck for only a short time before starting the swap and the brakes were fine. I had some leaks at new fittings but no leaks now.......finally!! I was thinking that maybe it is the "new" MC that I put in that might have a very small leak around a seal internally. I thought of doing the standard two man bleeding to force the liquid instead of gravity. Thanks again for your thoughts as well as the other members.
 

chengny

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Chengny. thanks for the response. Did a GEN III swap in my truck so this is new condition basically. I drove the truck for only a short time before starting the swap and the brakes were fine. I had some leaks at new fittings but no leaks now.......finally!! I was thinking that maybe it is the "new" MC that I put in that might have a very small leak around a seal internally. I thought of doing the standard two man bleeding to force the liquid instead of gravity. Thanks again for your thoughts as well as the other members.

So the condition began only after doing the engine change - correct? Any other changes made during the engine swap?

You mention this:

I had some leaks at new fittings but no leaks now.......finally!!

Does that refer to leaks at the brake fittings - ones that only developed after you changed all those brake parts you mentioned? Or does it mean vacuum leaks associated with the new engine?

Also, does the pedal go low with the engine running and the truck is stationary - or does it only happen when the truck is being driven?

Finally, if you started having this issue after the new brake parts were installed - is it possible the calipers are on the wrong sides? Do they look like this (i. e. with the bleeders at the top)?

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They can fit on either side. But if they are reversed the bleeders will be at the bottom of the piston cylinders - and the calipers might not bleed properly. A pocket of air can possibly remain up top.
 

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If you didn't bleed them AFTER you fixed the leaks . . . likely need to be bled again.
 

Heysq86

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Rotors are on correctly......bleeder on top. All leaks, on brake fittings, were fixed before this last attempt at getting a firm pedal. Leaks came up when I initially tried to bleed....actually as soon as I put fluid in the MC. Also, the pedal goes to floor when starting the truck.

Thanks again for suggestions.
 

chengny

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Rotors are on correctly......bleeder on top. All leaks, on brake fittings, were fixed before this last attempt at getting a firm pedal. Leaks came up when I initially tried to bleed....actually as soon as I put fluid in the MC. Also, the pedal goes to floor when starting the truck.

Thanks again for suggestions.

It's normal - and correct - for the pedal to fade slightly when the engine is started. Matter of fact, not doing that is the defining characteristic of a booster problem:

Check for proper brake booster operation
  1. With the engine "OFF", depress and release the brake pedal 4-5 times (this bleeds off the vacuum reserve in the booster).
  2. Depress the brake pedal firmly and start the engine. If the pedal goes down slightly the brake booster is assisting in brake operation.
If the pedal does not go down slightly the brake booster is not assisting in brake operation and should be diagnosed for problems.



But obviously, it should not fade to the floor. Look through the list below (it's from the GM service manual) and see if you agree that the only thing you haven't checked is number 7. It sounds like you have done a thorough job of inspecting everything else. That's why I mentioned a mismatched booster/MC earlier.

The other possible causes would either develop over time - as opposed to immediately after work being done on the system, would not be effected by the engine running (and powering the booster) or would cause a low pedal only while the truck was in motion.

EXCESSIVE PEDAL TRAVEL

  1. Worn brake lining.
  2. Shoe and lining knock back after cornering or rough road travel.
  3. Piston and shoe and lining assembly not properly seated or positioned.
  4. Air leak or insufficient fluid in system or caliper.
  5. Loose wheel bearing adjustment.
  6. Damaged or worn caliper piston seal.
  7. Improper booster pushrod adjustment.
  8. Shoe out of flat more than .005 inch.
  9. Rear brake automatic adjusters inoperative.
  10. Improperly ground rear brake shoe and lining assemblies.
 

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