Brake Pedal Flies to the Floor

[7.3IH_Eggrole]

Alright, so since what I'm about to described happened today I have been reading older posts about "sinking" and "spongy" pedals.

My problem is this with my 92 IDI:

I step on the pedal and it flies to the floor like I missed the pedal all together.

New calipers and pads a month ago, gravity bled them on install, checked them again the next day and then again a week later. No air the second or third time.

Rear brakes are both adjusted out pretty close to rubbing.

Vaccum reads 20inHg recovers within a few seconds.

My reservoir is full, and no noticeable leaks anywhere.

If I can get someone to sit in the truck and push the pedal tomorrow I am going to check and see if the level in the reservoir actually goes down when the pedal is pressed.

If the fluid goes down I'll probably bypass the RABS.

Any other ideas, the pedal really does go down fast.

 

[justinray]

Just because youre not losing dluid doesn't mean you don't have any leaks, remember, air is thinner than brake fluid, it will come in a smaller hole than fluid will leak out of.

 

[CurtIDI94turbo]

I'm thinking that the master cylinder is by passing itself, your orings maybe gone. Rebuild or replace it. just mt 2cents.

 

[justinray]

Also true^ but I would try the free checking of ALL fittings and bleeders first.

 

[riotwarrior]

betcha MC is shot...JM2CW

 

[Wyreth]

I have had several MC's pop on me and the only sign was when I pulled them off the boster. THEN it would puke out fluid that had gotten trapped between it and the boster. Nothing wierd or leaking before that. Just suddenly had no pedal. Scary when it happens pulling onto an offramp.

Pull it off and have a look.

 

[boxathey]

My Money is on the master cylinder I have had same thing and seen same thing happen on many different vehicles. and most of the time there is no visible brake fluid leaks at the mc.

 

[G. Mann]

Had that happen on my 85. Spent $$$ rebuilding the brakes.... ended up, the vacuum pump was no good and the brake booster wasn't working.... new pump, new belt, hard peddle.

Could be your problem.

 

[riotwarrior]

does not appear many betting on much other than MC...may wanna check MC since the collective is in agreement.......Next question for the IDIBORG!

 

[7.3IH_Eggrole]

Thanks fellas, no more than the parts are I'll tear into the MC. Any words of caution/wisdom for doing that?

 

[riotwarrior]

Thanks fellas, no more than the parts are I'll tear into the MC. Any words of caution/wisdom for doing that?

If it pukes when you pull it off the booster...go get a replacement and replace it...OH ya don't forget the bench bleed first...

 

[Matrix37495]

It could be the master, but you said you still have the RABS valve. If the dump valve in there is stuck open the it will do just what you're saying, and you wont lose any fluid.

Just another thought.

 

[Compu Doc]

Before throwing parts at it until the problem is solved do a little diagnosis first. Disconnect the brake lines from the MC and insert plugs that will not allow fluid ro pass through them. Then start the truck and step on the brake. If the pedal still goes to the floor replace the MC. If it does not then bypass the RABS and if the pedal is good then with the truck running replace the RABS.

One or the other being bad will give the same symptoms and by isolating them your able to save time and money.

 

[riotwarrior]

Before throwing parts at it until the problem is solved do a little diagnosis first. Disconnect the brake lines from the MC and insert plugs that will not allow fluid ro pass through them. Then start the truck and step on the brake. If the pedal still goes to the floor replace the MC. If it does not then bypass the RABS and if the pedal is good then with the truck running replace the RABS.

One or the other being bad will give the same symptoms and by isolating them your able to save time and money.

Outstanding suggestion and one I have done....just completely slipped mind to do that...usually pull MC off booster first for signs of leakage...instead of crack open the brake lines...but that's me...

 

[7.3IH_Eggrole]

Thank, I'll keep yall posted. I might try the plug idea after I get home