Problem: Rear brakes locking up first in heavy braking

Eric86Red911 · 04-29-2003, 01:12 PM

Ok, so that's the problem, and here's my guess at possible causes:

1. air in front lines (if i should be so lucky)
2. old squishy brake lines (ditto)
3. bad master cylinder, requiring replacement ($).
4. caliper problems up front, requiring rebuild ($$).

Can the caliper and manifold be somehow checked before doing rebuilds? I'm a little worried that the tech might just take the shotgun approach and replace everything.

Eric

JWest · 04-29-2003, 01:25 PM

A couple more:

-Contaminated pads
-Failed proportioning valve

-armor-all on your tires

ChrisBennet · ChrisBennet's Garage

That sounds exciting!
I'm not recommending replacing everything to fix your problem but if your brake system is over 15 yrs old you might consider replacing stuff just as a safety issue.
Brake stuff is not that expensive if you do the work yourself. If you still have your original brake lines, now would be a good time to replace them with some nice new (rubber) ones. Master cylinders are not that expensive either.
-Chris

Neilk · 04-29-2003, 01:35 PM

Have you checked the tire pressures? Are all the tires in decent condition? Has anything changed recently that might have affected the brakes, eg new brake pads, brake fluid change?

Are you willing to do any of the repairs yourself? It's really not that bad a job if you have another car at your disposal.

Eric86Red911 · 04-29-2003, 01:37 PM

Yup, it's pretty exciting. Just like stopping on a Big Wheel. A really big one, that is.

Eric86Red911 · 04-29-2003, 01:38 PM

I put new tires on it (S-03), and it's still locking up in back. No other changes.

Early_S_Man · 04-29-2003, 02:26 PM

Eric,

How recently has your brake fluid been changed ... whole system cleaned, rinsed out with alcohol, and blown out with air in the case of the steel hard lines? And, was the brake fluid really grungy when you bought the car?

I would place my wager on the proportioning valve ... due to corrosion build-up because of moisture accumulated over a number of years!

Incidentally, among road tests of all 911/912 cars prior to 3.2 Carreras ... it was quite common to read complaints by testers about the right rear wheel locking up prematurely! That was before a proportioning valve was installed, of course!

Eric86Red911 · 04-29-2003, 04:52 PM

Warren,
Thanks for the tips. This will be my first brake bleed since owning the car, so I have no idea about fluid condition.

I'm really worried that there doesn't seem to be a way to positively identify the culprit before replacing everything. Starting to sound expensive...

Early_S_Man · 04-29-2003, 05:17 PM

Eric,

In the case of unknown fluid condition, my standard recommendation is to rebuild all four calipers and master cylinder, then replace the flex lines with new OEM rubber hoses ... just to get the brake system back to a standard level of routine maintenance. Due to rust, dirt, and dust from the pads built up over several years ... your caliper pistons could be binding, and may not be all pulling with equal force side-to-side, hence a minor variation in forces could cause the lock-up problem.

Eric86Red911 · 04-29-2003, 05:23 PM

DOH! Sounds like $1500.

Early_S_Man · 04-29-2003, 05:35 PM

Eric,

Not really, if you do the work yourself! Probably less than $250 in parts, including a tube of Dow Corning 111 silicone grease.

dad911 · 04-29-2003, 05:37 PM

Quote:

Originally posted by Eric86Red911
DOH! Sounds like $1500.

maintenance or bodywork. your choice. Just please don't tell me you track the car with those brakes. I once had a student that didn't understand the importance of fresh brake fluid until the end of the back straight at Watkins Glen.

Eric86Red911 · 04-29-2003, 06:41 PM

ummmm, well, yes I'm planning to track the car. Road America, at that. But I'm smart enough to know to get the brakes fixed. Probably too busy to do it myself tho...

Don Wohlfarth · 04-30-2003, 12:15 PM

I mostly agree with Warren up to a point.

I would get a power bleeder and a couple cans of cheap brake fluid and flush the system throughly. Before the flush I would remove 4 rubber brake lines and check the condition. If unknown I would replace these 4 lines, then flush system. Just look and see what the old brake fluid looks like, little pieces of dirt, cloudy, who knows. If you are comfortable that the fluid is OK fill brake system with Super Blue or comparable. If theres a bunch of dirt and crap in the lines rebuild the calipers, flush, install Super Blue.
There may still be a problem with proportioning valve but it's a fixed orfice that could be come clogged cutting down the pressure to the rear brakes, not increasing it.
You may also have unknown brake pads front to rear. Somewhere along the line someone could have installed competition pads on all corners, wore out the fronts, and installed a stock set in front leaving comp pads in rear.
This kind of stuff shows up when you start putting 15-20 year old cars on the track and you start pushing it. When you drive on the street it's not a problem.
None of this is any big deal.