Excessive pedal travel on rebuilt S callipers, but car stops fine Options

[phillstek]

Just finished rebuilding the front callipers with new pistons and seals. Bled them 3 times including power bleeding but the pedal just won't come up. It feels like half the available pedal travel is in free motion before the brakes engage.

Car stops without pulling or locking up and the pedal pressure holds when stomping on them but I would prefer to have the pedal engage pretty much as soon as I hit them.

Any suggestions on possible cause and what I need to do to fix it?

Thanks

[mepstein]

Have you bled the MC?

['73-914kid]

Is it a nice firm pedal once they do engage? And is it a freshly rebuilt pedal cluster?

There is an adjustment at the brake pedal to extend the arm into the MC a little farther. If you have a really solid pedal, that is where I would look.

[904svo]

Does the brake pedal return all the way? The brake light switch may be stopping it now.

[phillstek]

Thanks for the replies.

Have you bled the MC? Yes.

Is it a nice firm pedal once they do engage? And is it a freshly rebuilt pedal cluster? Yes and yes.

Does the brake pedal return all the way? Yes

I rebuilt the rear callipers about 2 years ago and replaced the brake lines, re-bushed the pedal cluster and installed a new b/f reservoir as well. I don't know why but I left the fronts callipers alone. Brakes were great for the first 250 miles of driving with proper pedal feel etc. The pedal went bad when taking the car for an extended road test and required pumping to get pressure and the right front was wanting to lock up when applying the brakes hard.

Determined that both fronts were leaking and discovered some incorrect pistons had been installed earlier by someone unknown. Got some correct replacement pistons from PMB, put those in along with new seals, pads and rotors.

The car pulls up really well without pulling left or right it's just that micro second of doubt when hitting the brakes and thinking "are they there"

I'll do a full inspection on the weekend and see if I can work it out.

[THE STIG]

I just ran into the exact problem, went from a Brembo 4 piston to 911S calipers on my Vintage Race Car. No rebuild on the S caliper, but exact same problem. Pedal with the Brembos was rock hard and immiediate. Switched to the S calipers, had to adjust brake plunger inward to get the same pressure point for heel/toe purpose, and now have to bleed the brakes after every 30 minutes session. They get bad. Have brake cooling as well, so I know its not OVER heating. I know the previous owner installed stainless steel pistons, and car was for AX use only. Rarley had to bleed them with the Brembos. Maybe after a test, qualify and race, they would need to be bled. They do stop the car, stops straight, but why the pedal feels different is a mystery. Its possible that the known flex in the caliper body is the cause of this, but you can feel it sitting in the car, stopped with no pressure applied. Pedal travel increased because of a caliper makes no sense. I also experienced great difficulty with bleeding the 911S caliper. Fluid runs air free while bleeding, but they just never feel good. Most folks say that is just they way thay are. What I do know for sure, is that I could run anything else on my car, I would.

THE STIG

[laflaur]

Did you check the venting clearances on the rears yet?

[THE STIG]

Did you check the venting clearances on the rears yet?

I think you missed it...only change made to the car was the front calipers, put them on and the pedal travel increased. Not a little, allot. Rear brakes untouched, already set.

[laflaur]

He is bleeding brakes right?
he emptied the system of fluid?
He should be bleeding the entire system,no?

[bulitt]

17mm or 19mm MC?

[phillstek]

He is bleeding brakes right?
he emptied the system of fluid?
He should be bleeding the entire system,no?

Yes, entire system has been bled 3 times,

I haven't had a chance to take it for a decent drive after the rebuild. I've only driven it around the block a couple of times. I'm going to take it out for thorough road test later in the week.

Thanks again for the replies.

[Eric_Shea]

Any suggestions on possible cause and what I need to do to fix it?

Yup:

1. Take out the knockback mechanisms.
2. Check the pad clearance on the S-Calipers. New seals will pull the pistons back aggressively. To verify, check the clearance of the pad and rotor.
3. Bed the pads. Multiple times. This will be good for the pads, rotors and the seals. It will help break in the seals.

Once the seals begin to break in your pedal will come back up.

[sixnotfour]

(IMG:style_emoticons/default/sad.gif)

[phillstek]

Any suggestions on possible cause and what I need to do to fix it?

Yup:

1. Take out the knockback mechanisms.
2. Check the pad clearance on the S-Calipers. New seals will pull the pistons back aggressively. To verify, check the clearance of the pad and rotor.
3. Bed the pads. Multiple times. This will be good for the pads, rotors and the seals. It will help break in the seals.

Once the seals begin to break in your pedal will come back up.

Thanks Eric,

I'll do that, though I didn't use the knockbacks that you sent me.

[brant]

eric beat me to it...

knock back mechanisms

I vintage race on S calipers and mine are removed.

sometimes the initial pedal is still long
but once you hit it, no more "flex" feeling
I honestly believe that all of the hype over "flexing" is really the spring effect of the knock back mechanism and mis attributed all of these years.

[rhodyguy]

guys...easy stuff first. haynes, p110, fi. 8.4. "Adjusting the piston rod free play".

[Cap'n Krusty]

Put a few hundred miles on it. BTDT, a LOT of times.

The Cap'n

[Eric_Shea]

Put a few hundred miles on it.

(IMG:style_emoticons/default/agree.gif)

[THE STIG]

eric beat me to it...

knock back mechanisms

I vintage race on S calipers and mine are removed.

sometimes the initial pedal is still long
but once you hit it, no more "flex" feeling
I honestly believe that all of the hype over "flexing" is really the spring effect of the knock back mechanism and mis attributed all of these years.

Opened them up tonight, found these knock back mechs in place, and a rolled piston seal. That'll do it. Ordering kits from FPS tomorrow.

Who are you running with? Just did my first HMSA Club Event at Laguna. I had a blast.

THE STIG

[brant]

eric beat me to it...

knock back mechanisms

I vintage race on S calipers and mine are removed.

sometimes the initial pedal is still long
but once you hit it, no more "flex" feeling
I honestly believe that all of the hype over "flexing" is really the spring effect of the knock back mechanism and mis attributed all of these years.

Opened them up tonight, found these knock back mechs in place, and a rolled piston seal. That'll do it. Ordering kits from FPS tomorrow.

Who are you running with? Just did my first HMSA Club Event at Laguna. I had a blast.

THE STIG

RMVR, CVAR, and HMSA

I hope to be at the June legends event at laguna
send me a pic of your car?
what are the specs and details of your car?
it would be great to meet... any chance your going to the June Legends event?

brant