Adding proportioning valve to brakes?

[Flying Toaster]

Car: '74 911 with fairly new MC, stainless lines, rebuilt "M" calipers, Mintex pads in front, Hawks in rear, 15x7's & 8's, super blue.

I have been having trouble with the front brakes locking up under reasonably hard braking, mostly during autocross events. I tried going with more aggressive pads in the rear, but after one autox event, I don't notice a difference. I have the front apart right now rebuilding the calipers which needed it and reinstalling with new rotors.

My next thought was to install a Tilton or Wilwood brake proportioning valve to limit the feed to the front. If this has been done before, I would appreciate feedback as well as part numbers used. (I noticed they come with different sized 'an' fittings).

Thoughts? Thanks in advance. -John

[Elombard]

Proportioning valves are ONLY supposed to be on the rear. Generally you want to raise the performance in back to match the front. I dont think the pre 84 cars had a pressure limiting valve on the rear that you can remove.

I had an issue like you describe when my corner balance was off. I suspect you have a balance issue. Tires not matched or sized wrong, worn or mismatched shocks, Corner wieight off, worn bushings, sagged T bar etc. etc. can all play a role.

[reddogmotrsprts]

Your brake bias is already ideal, assuming you are running an S caliper up front. I'd see what your changes yield and then start measuring temps at your rotors. Sounds like you have a problem; are there any other symptoms or details you can provide? The brakes are supposed to lock up given enough force.

[Flying Toaster]

I will forget the prop valve idea.
The issue occurs whether I use my street tires 205/50 & 225/50 or the autox tires, 225/45 & 275/35.
I'm sure one contributing factor is weight loss in the front when I went to RS FG front bumper and FG front fenders.
Of course I just had the alignment done last year once I thought I was done with suspension mods (ie bushings, bearings, T-bars). I don't have the alignment specs off the top of my head, but I recall maxed out caster & camber [-1.5/-2.5], toe'd out & in, etc..
FWIW, my T-bars are 21/30's. Shocks are Boge's.
I will get the brakes back together and re-evaluate. Perhaps it was just a caliper hanging up all along.... fingers crossed.

[Bill Verburg]

Originally Posted by Flying Toaster

I will forget the prop valve idea.
The issue occurs whether I use my street tires 205/50 & 225/50 or the autox tires, 225/45 & 275/35.
I'm sure one contributing factor is weight loss in the front when I went to RS FG front bumper and FG front fenders.
Of course I just had the alignment done last year once I thought I was done with suspension mods (ie bushings, bearings, T-bars). I don't have the alignment specs off the top of my head, but I recall maxed out caster & camber [-1.5/-2.5], toe'd out & in, etc..
FWIW, my T-bars are 21/30's. Shocks are Boge's.
I will get the brakes back together and re-evaluate. Perhaps it was just a caliper hanging up all along.... fingers crossed.

It's most likely an issue w/ the rear brakes, one common issue is the rubber brake lines that can swell internally and inhibit rear brake torque, another is one or more frozen pistons.

[Elombard]

I am assuming with all those awesome bits they did the ride ht and corner balance. If not maybe check it with the tri pod method in the garage real quick. For what you are doing it just needs to be close. Yeah I bet there is a seal or something in a caliper that is hanging. Maybe the new Master Cylinder has a problem? I have seen that (not on a p car).

[ted]

Here's how I use the brake bias on my non 911 race car.
I have the twin master cylinder with the balance bar brake set up.
On track during continuous laps (not AX) I start off with more front brake bias.
After about a lap or two the rear tires warm up and I add rear brake bias.
If I go out with too much rear bias the cold rear tires the hot rear brakes will lock the cold rear tires until the rear slicks get hot.
Seems my rear brakes get heat in them before the rear slicks.

[Porsche 2]

In my similar case it was the master cylinder. No further problem after replacement.

[stlrj]

Originally Posted by Flying Toaster

... I don't have the alignment specs off the top of my head, but I recall maxed out caster & camber [-1.5/-2.5], toe'd out & in, etc..
.... fingers crossed.

Reduced front end tire patch due to maxed out camber may be the reason your front end locks up so easily. Increasing tire patch might be all you need to do since braking ability has a lot to do with being able to put down enough rubber to do any good.

Cheers,

Joe