mrpakchong

Hi,

I have tried to bleed my brakes,
All 4 brake calipers are new, the M/C is new
I'm using a A Motive bleeder (very nice)
The M/C has been bleed before I applied it to the booster

The bleeding sequence I'm using is: LeftRear inner, LeftRear outer, RightRear inner, RightRear outer, RightFront and finally LeftFront.

The brakes are quite hard until I start the engine, then the pedal go easy down to the floor.

Any one have any hint what to do next ? . Could the booster be faulty ?.

Thanks for any help

Matt Gruber

sounds good so far.
time for test drive!

big_G

You will notice a small drop in pedal height as you start the engine. It should not go to the floor. Does the brake warning light come on when you press the brake pedal hard, with the engine running? If not, and you have a couple of inches of clearance to the floorboard, drive it and see how it feels. You may still need to bleed the brakes if the light comes on.

turbovette80

same for me, all new parts, good pedal until engine starts then it sinks to floor.

mrpakchong

Thanks,

I will check the lights tomorrow and take another round with the bleeding, and also disconnect the vacuum line from the booster to see how the engine runs.

White79

Chong,
Disconnect the vacuum lines to the Vacuum booster and plug them with your finger. If you get a good suction then its probably not the booster.
IMO

~Steve

PNK

I have had this problem many times when converting from manual to power brakes, i can get a good firm pedal until i start the engine then the vacuumbooster pulls the pedal to the floor, the car brakes fine but the pedal is just to close to the floor, i just fabricate a longer pushrod from the brakepedal and it solves the problem.

GaryS

In my world, these are indications of a failed master cylinder. Since you said you already replaced the MC I would re-bleed the MC and then do another round of bleeding on all four calipers. Are you sure that all the bleeder screws are in tight after bleeding. Did you fill the MC after each caliper was bled?

big_G

The booster is only an assist device. It is a straight connection between your foot and the master cylinder. If the brake pedal is too low when the engine is running, you have a problem with either air in the system, or the master cylinder could be leaking internally. Another thing to check is the booster pushrod adjustment. You want a little free play, maybe .030 to .060 inches.

Matt Gruber

does it stop straight?

TPI BOY

How do you ajust the pushrod can you do it with the
booster still mounted on the car

bobs77vet

sounds like air in the system to me.

TPI BOY

ttt

bobs77vet

i adjusted the pushrod on my 69 last night......i had the seat out and crawled in head first to where the pedals are and you have to loosen a lock nut onthe push rod shaft and then turn the push rod to make it the length you want and then tighten up the lock nut.....its not technically hard but getting at those spots is really tough.....i had to use a long needle nose set of pliers to turn the push rod about 1/16 of a turn at a time to move it.....my back is still hurting ...i dont know how you could do it with the seats in

big_G

The adjustment for the power brake booster rod is usually at the end of the booster pushrod. You need to unbolt the master cylinder and move it away a little. No need to loosen the brake lines. Be absolutely sure of proper bleeding of the system before making any pushrod adjustments.

TPI BOY

Who's right ajust from inside or outside or can you
do it both ways

big_G

My Assembly manual and service manual list the brake pedal free-play (manual brakes) is adjusted to .25 inches. Adjustment is at the clevis under the dash. Power brake pedal free-play adjustment (.010 inches) is at the rod end between the master cylinder and the power booster.

Roughrider

No one else has mentioned it, but it seems to me you're bleeding your brakes in the wrong sequence. You have the rear reversed...it should be right rear then left rear, then right front and finsihing up with the left front. You start with the furthest from the master cylinder to the closest.

It could be as simple as that...some air still in the line due to bleeding out of sequence.

0Corvette Engineering

I disagree.
Vacuum boosters are always spongy. Every car I've been in with the engine running I can put the brake pedal to the floor.

A little spongy is ok on a power system. the best test is to drive it and see how it feels then. Make some easy stops and then harder stops.

You'll know if it's working

TPI BOY

Are you sure thats right