Soft brake pedal when engine running
Thread Starter
8th Gear
Joined: May 2007
Posts: 8
Likes: 0
Soft brake pedal when engine running
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
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
Last edited by mrpakchong; Jun 3, 2007 at 04:09 PM.
Le Mans Master
Joined: Jul 2005
Posts: 5,752
Likes: 4
From: Austin Texas
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.
Instructor
Joined: Feb 2006
Posts: 144
Likes: 0
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.
Melting Slicks
Joined: May 1999
Posts: 2,897
Likes: 0
From: Moon Township PA
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?
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Le Mans Master
Joined: Jul 2005
Posts: 5,752
Likes: 4
From: Austin Texas
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.
Burning Brakes
Joined: Dec 2002
Posts: 1,179
Likes: 3
From: STANTON CA
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.
booster still mounted on the car
Last edited by TPI BOY; Jun 6, 2007 at 03:43 PM.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
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
Le Mans Master
Joined: Jul 2005
Posts: 5,752
Likes: 4
From: Austin Texas
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
Burning Brakes
Joined: Dec 2002
Posts: 1,179
Likes: 3
From: STANTON CA
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.
do it both ways
Le Mans Master
Joined: Jul 2005
Posts: 5,752
Likes: 4
From: Austin Texas
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.
Melting Slicks
Joined: Mar 2007
Posts: 2,100
Likes: 12
From: Frederick Maryland
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.
It could be as simple as that...some air still in the line due to bleeding out of sequence.
Former Vendor
Joined: Nov 2005
Posts: 683
Likes: 1
From: Mounds View Minnesota
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
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
Burning Brakes
Joined: Dec 2002
Posts: 1,179
Likes: 3
From: STANTON CA
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.
It could be as simple as that...some air still in the line due to bleeding out of sequence.