Brake Pedal going to the floor

76Vett · November 23, 2014, 07:27:00 AM

Good morning All. I need the help of the brains trust once again.
Ok what I have done so far on my brakes:
Had the disc's machined
Replaced master cylinder (and yes primed/bled it on the bench first before installing)
Replaced brake lines and hoses
Replaced all four callipers and pads
Primed and bled all lines and callipers with a pressure bleeder
Bled air from calliper pistons by pumping the pedal
Have repeated the bleeding process four times thus far
The pedal is hard and feels good without the engine running
The problem is that the pedal still goes nearly all the way to the floor when the engine is started ie. vacuum applied to booster
There is some braking but minimal

I am now at a loss of what to do or what the problem is, any clues

bfit · November 23, 2014, 08:33:06 AM

Do you have the corect master cylinder for the model you have
Bfit

sirfixalot · November 23, 2014, 09:33:05 AM

Around 76 they changed the length of push rod/piston recess depth also check that all pads are straight heard of a similar situation where a pad had a bow in it which straightened out when piston pushed against it. Also try jacking car up on each corner you are bleeding Rrh 1st Rlh 2nd Frh 3rd Flh 4th also tap lines & caliper with a mallet to move air along pipes, calipers

77CVT · November 24, 2014, 09:35:43 AM

Also, if you want to determine which caliper may be a problem, clamp off all 4 lines and put your foot on the pedal. Have someone release a clamp and feel how far the pedal travels (usually 10-20mm). Foot off the pedal, reclamp and repeat process for the other corners. If one is bad, you will feel a huge difference in travel compared to the others.

If it's the push rod, you won't need to adjust it by far.

sirfixalot · November 24, 2014, 05:27:38 PM

Is master cylinder 1' or 1 1/8should be 1 1/8"

76Vett · December 30, 2014, 10:11:44 AM

Hello again. Thanks for all your tips. I gave up on this problem for a while and have been fixing other things. I decide to give this another go yesterday with some assistance from my son. I tried all the tips and tricks that you all mentioned and did get some more air out of the lines after bleeding another 3 times. The pedal feels very firm now with the engine off but again once the car is started the pedal has a lot of travel, probably 100mm (too much for my liking). This morning I double checked the length of the Booster push rod in relation to the depth of the recess in the Master cylinder piston and both seem to match at 1 1/8". What am I missing?

toyvet · December 31, 2014, 09:31:28 PM

HELLO 76 VETT FOR THE ANSWER TO YOUR BRAKE PROBLEM CALL ( ALLAN DEAN ) CLUB MEMBER ON 0430769293 HE'LL BE HAPPY TO HELP YOU WITH YOUR PROBLEM.
REGARDS

JORGE

bfit · December 31, 2014, 11:06:10 PM

Question

Have you access to the equipment to make a brake line adaptor to fit a gauge inline to measure exactly
What pressure brake system is developing.
Check both circuits your looking for 1000 to 1200 PSI

Have you checked to see what happens at the master cyl. When the brake peddle released
Take the lid off and watch as the brakes are applied and released ,
Is the oil is flowing back to the resiviour with some force , then there is a problem down stream.

If the flex hoses are new quality hoses then there is still air in the system.
Bfit

72vet · January 01, 2015, 11:44:16 AM

And another question,

do you still have the old master cylinder, if so, compare the recess depth of the old master cylinder piston to the new one. Are they the same?

76Vett · January 03, 2015, 06:40:39 AM

Thanks to all for your tips and guidance. I will try your suggestions and see how where I end up.

76Vett · January 15, 2015, 08:02:27 PM

Ok, that's it, I officially give up.... throw may hands up in the air.... throw in the towel.... admit defeat.... I've been beaten

I have tried everything, given all your tips and hints a go, tried things from the University of Google and still no usable brakes. Must be time to hand them over to an expert. Starting to suspect the new calipers might be faulty some how, don't know how or why but it's the only thing left.

toyvet · January 15, 2015, 09:56:17 PM

:pepper:OK 76 VETT -- NOW THAT YOU HAVE OFFICIALLY GIVE UP -- CALL STEVE AT SR PPERFORMANCE ( ON 95974048 ) AKA THE VETTE DOCTOR LOL HE"LL FIX YOUR BRAKES AND ANY OTHER PROBLEMS THAT YOU HAVE WITH THIS CAR.

GOOD LUCK

REGARDS

JORGE ( CLUB PLATE REGISTAR )

bfit · January 15, 2015, 10:04:11 PM

Quote from: 76Vett on January 15, 2015, 08:02:27 PM
Ok, that's it, I officially give up.... throw may hands up in the air.... throw in the towel.... admit defeat.... I've been beaten

I have tried everything, given all your tips and hints a go, tried things from the University of Google and still no usable brakes. Must be time to hand them over to an expert. Starting to suspect the new calipers might be faulty some how, don't know how or why but it's the only thing left.

Your not that far away . If you want a hand to go over it just PM. Me
Bfit