Brake caliper drag

jacko241 · 09-07-2006, 01:39 PM

I have 993 calipers front & rear on my 77 911 race car. The rears always spin freely with no extra drag. The fronts however, have a significant amount of drag, which gets worse when they are hot. I would normally assume that they need rebuilding, but all 4 pistons on both sides go in smooth as silk. Is it still possible that rebuilding them would improve this, or is it likely something else i'm not thinking of? Maybe some of you brake gurus can help!

Bill Verburg · Bill Verburg's Garage

First thing that come to mind is that there is residual pressure in the lines for some reason.

likely suspects
brake lines
m/c
reservoir vent

jacko241 · 09-07-2006, 02:47 PM

Bill, thanks for responding. Being that it is both sides I was thinking it may be something with the master cylinder. Would a problem with the reservoir vent cause the master cylinder to maintain residual pressure?? I haven't actually tried to open a bleeder and see if that relieves the pressure.

Bill Verburg · Bill Verburg's Garage

The res. is easiest to check, just take the cap off, there should be a flexible membranethat allows for expansion and contraction of fluid vol. in the res.

If you have rubber lines they will wvwntuallt swell internally and retain pressure in the caliper.

A blocked port in the m/c can prevent fluid from escaping to the m/c

damaged or miss seated seals at the pistons can also prevent full release.

LakeCleElum · 09-07-2006, 03:30 PM

When that happened to my '73, it was the flexible brake lines. When I got the fronts off, I couldn't blow thru them with an air hose...

jacko241 · 09-07-2006, 03:55 PM

I am going to investigate further, and will report back. Thanks, guys.

hcoles · 09-07-2006, 05:33 PM

my observation is this - rebuild brakes, install new seals...pistons retract nicely....do track days heat the crap out of the brakes, boots turn into potato chips and I guess seals get hot also..... pistons don't retract very well..
my 2 cents is that when you cook the seals to a certain point they don't aid in the retraction like when new..other threads cover how the seals help retract the pistons

jacko241 · 09-08-2006, 03:38 AM

HC, that is very possible. I am going to check out everything. How difficult is it to rebuild 993 4 piston calipers?

MikeBogue · 09-08-2006, 07:31 AM

If the pads/pistons retract smoothly then the problem is not likely to be the rubber lines. If they go bad they will close up the passageway as LakeCleElum said, but if they are plugged they will not move back easily and will bleed badly. Since it is on both sides I would suspect one of two things wither a problem with the master cylindar causing it to partially close off the return. Also the dust shield if either installed incorrectly, or if they get old can get stuck between the piston and pad and that can cause the symptoms you have.

hcoles · 09-09-2006, 08:18 AM

I don't know re. 993 calipers..the 89 3.2 calipers are straight forward except for getting the pistons out...might be able to use 2 screw drivers and getting the rear caliper piston boots on...I hear using a flat piece of wood and compressing evenly is a way.. the fronts are easy because of the separate spring... I cleaned and buffed the pistons with 800 grit and cleaned the cylinder side and did some scrubbing there also to knock down any stray material or burrs. The PITA part is dealing with all the undoing of brake lines/etc. Make some plugs with tygon tubing and screws put in one end...these go over the line ends as you undo. I did complete brake jobs on my 3 cars this last year and I'm sick of brake fluid