Maxhouse97

This is on a pair of 996 front calipers. I plan on swapping the cross over tube and bleeder valves, so the bleeders are still pointing up. I am trying to diagnose a right front wheel that locks up before the left and whether the problem follows the caliper.

I think the calipers are the same other than the cross over tubes.

Keith Verges - Dallas

Unless the Pistons are staggered for taper wear that should work. Have you checked corner weights? A light front will lock first.

Texas RS

...before you do this, what troubleshooting have you done?

lordpantsington

A few months back I was examining the Brembo racing catalog and found a trend. All calipers had pistons arranged by size smallest to largest such that a rotating disc encounters the smallest piston first (going forwards). I assume this is optimal for combating taper, as Keith suggested.

audipwr1

those calipers are actually pretty cheap - reman is like 400

trygve

On the 986 (SPB) front calipers at least, the pistons are staggered. I expect the 996 are very similar.

Maxhouse97

Yes the pistons are staggered - good point. So that rules it out for the long term, but I wonder if I could do this for a short term test?

Troubleshooting completed:

Bled brakes
Changed fluid
Changed pads
Professional corner balance
Watched Pistons extend while someone pressed the brake pedal (this is where I noticed a piston on the side that doesn't lock up extending slower than the others)
New master cylinder

Of course it could be other than the calipers, but the lines from the car are relatively new SS braided. I bought the brakes used.

I want to isolate the problem. Yes if it's the caliper, I might buy a remaned one. We will see.

Maxhouse97

And I forgot even though I had the car corner balanced with a weight in only the drivers side seat, I've checked with a ~190 pound passenger and the right still locks up first. Any other ideas for troubleshooting?

linzman

Don't know the answer to this, but In an ABS car, why does one side lock up? Is there a problem with the ABS sensor?

If you can see the opposite side pistons extending slowly, seems to me like there is a pressure difference between the 2 sides (small leak??) or a mechanical issue with that piston. Maybe it's time for a rebuild? Relatively quick and easy to do yourself.

You could isolate it to that caliper by temporarily swapping sides, but be careful. It's really easy to strip the threads when switching the bleeders and the crossover tube. Ask me how I know And if it is that caliper, which seems likely, and you do rebuild it, you should rebuild the other one too.

bobt993

Def check the typical issues up front. wheel bearings both sides for pad knock. Sway bar bushings and anything loose. A glazed rotor can cause this when hot too.

car_slave

"Watched Pistons extend while someone pressed the brake pedal (this is where I noticed a piston on the side that doesn't lock up extending slower than the others)." Bad caliper is the one that's not locking - extra force goes to opposite side. Rebuild kit is cheap.

I'm not sure what the conditions are when this is occurring but the right will always lock first on any left-hand drive car.

Frank 993 C4S

How old are the calipers? When were they last rebuilt? Does your car have ABS?

Maxhouse97

OK sorry I didn't clarify upfront:

- These are 996 brakes on a 87 - no ABS
- Agree bad caliper is on the side that does NOT lock up (left side) ... this is the side where the piston is slow to extend, hence my thought that this caliper is not exerting the same amount of force as the right side, which is locking up first. Make sense?
- Agree it may be due to a bad seal … I have read that if there is a slight leak from a bad seal, the piston will tend to retract a little into the bore, which might cause the lack of force. I bought the calipers used, so I don’t know their history, but they very well may have sat on a shelf for a while unused which hurt the seals. I can rebuild myself, my only hesitation was if I do that work myself, but them back on and still have the same problem. Hence the desire to isolate to the caliper. Also is it’s a bad piston and or bore I’m not sure that’s something I can determine myself on inspection (unless its obvious).
- Rotors are in real good shape, don’t think they are glazed. Wheel bearings are new, everything else looks tight. Main point is that when the stock brakes were on the car I did not have this problem, which leads me to believe it’s the caliper or somewhere upstream (proportioning valve? … although that should be front to back, not side to side).

Not sure I agree a left hand drive car will always lock the right side first. As I mentioned above, I don’t think that was the case before (it was a while ago). Anyone else care to chime in?

The reason this was a problem was on the track I was setting up for a right hand turn, and prior to turn in I was hugging the left side of the track. Upon braking and locking up, my car pulled to the left (right side locked up) which drove my left side tires into the grass. Not good.

I’m going to call another race shop to see what they would charge to rebuild and TEST the caliper properly (right amount of force being applied by the pistons, etc.) … something again I can’t do. If that’s too expensive, I guess I will buy the seal kit, rebuild them and hope for the best. If they are still AFU I guess I’ll buy another set of used and go from there. Again just hate to dump money into these calipers if that is not the root cause…

NaroEscape

doing the rebuild yourself is easy and cheap - unless there are bad pistons (very unlikely...they are just a cylinder of metal and which you would see immediately if there was an issue) its just cleaning and replacing seals.

I've done all the calipers on all our cars (race cars, street GT3) and was surprised how much 'crap' was in the caliper - at some point water must have gotten in the system and even with multiple full brake flushes, there was still milky fluid in a couple.

since you had planned to take both calipers off anyway to swap them, just take them off and rebuild them...

Frank 993 C4S

Bingo!