Ok, here it is:

The Situation:

- bought a complete '86 911 Carrera Front Suspension about 2 years ago.
had a shop put it in. also upgraded the MC to 911 19mm (early style 69-77 i believe).
improved braking a lot over the stock brakes but it had too
much bias to the front, the rear brakes didn't do nothing.

- replaced stock prop-valve with a "Tee". still stock /4 brakes in the rear,
braking got better, moah thumb in the rear, but under hard braking, the
front still locks up way to early.

- today, went and got '87 911 Carrera rear rotors and calipers and put them on.
big fu***ing HUGE calipers, vented and cross-drilled rotors and all.
put them on, bled the brakes, nice, firm pedal, test the rears. they work great.
get on the road for a test drive, and WTF?
the front STILL locks up WAY before anything else. you can feel the rears working,
the car goes down as a whole, no nose-dipping. same (street) tires all around, in
good condition.

The Question(s):

- Porsche changed the 911 MC in '78. i have the 19mm MC from a earlier 911 but
now i'm running 86/87 carrera brakes all around.

- What is the difference between the 2 types of 911 MCs ???

- does the later 911 MC have different valving that gives you more "thumb" to the
rear brakes?

- is there a "reverse" prop-valve that can limit the FRONT brake?



Brad and i sit here scratching our pretty heads ...

anyone? PLEASE!

Andy

The question to ask is if the later MC is a stepbore.

Answer maybe tomorrow.

In any case, the fronts better lock up first for safety. Wild ride if the rears lockup first.

How 'bout use of a manual brake bias valve - not cheap - but............,

maybe, but not THAT MUCH ...




but all adjusteable brake bias valves i have seen allow you to control how much flow
goes to the REAR. i don't need to restrict the flow to the rear, i need to restrict the flow
to the FRONT.

all i would do with a normal brake bias valve is run it WIDE OPEN to get as much to the rear
as possible. that is NO DIFFERENT than running a "Tee" ...

Andy

Maybe there is some other issue -- alignment or sticking caliper? -- causing the front lockup?

Does it lock consistently on one side vs the other?

Pretty weird as I have SC brakes up front and stock 914 rears, with a "tee", and I have borderline perfect balance i.e. the rear "just about" wants to lock same times as fronts if I really hammer it. I have equalized this fairly well with pads; Pagid black (higher friction) front and orange (harder, slightly lower friction) rear.

Which raises other questions -- tires and brake pads??? Lines all in good condition?

The right front locks up about 3 feet before the left front. We know something is amiss there but it wouldnt cause what I'm feeling. I'm running several cars out there right now with 944 Turbo front and Carrera rears on vented rotors with a 19mm MC with NO issues. The pedal drops for perfect heal toe.... but Andy's has a nice firm pedal with no "mush" at all. It bleeds perfect every time. Lots of fluid from the rears.

The tires all have the same amount of mileage on them.

I do question the brake lines.

I may have to "stagger" the front and rear brake compounds.


B

Do you have new rotors & pads in the rear and "bedded in" gear in front?

I did something similar to this last year. I don't get much milage on my car. The rears really only came up to snuff after a 5 session DE.

After this years mods, I had to bleed the brakes 3 times to get all the air out of them......each bleeding a week (or so)apart.

AIR, the 77 S went to a vacuum boosted MC. The booster got bigger in 84

Because the heavy end of the gas tank is right over that wheel maybe? Has the car been corner balanced since it's latest round of "work"? Just brainstorming. 3 feet aint much.

-Ben M.

That was my thought, Start by replacing the rubber lines. As they break down they can close up inside. Guys start checking and changing all kinds of other things first. It can cause the uneven lockup in the front as well. No real way to check this without changing them.

What type of pads are on it?

I agree with the plan of dealing with this by varing pad compound.

I've seen this issue hundreds of times on some of the lesser cars I work on.

Andy, the club says: change the brake lines.


B

The Club has spoken!

but the club also said, put shitty brake pads on the front and good ones on the back.
Andy

I say keep driving it. JP nailed it with the old/new thought also.


B

Hmm, when you put it that way it seems so backyard.

Actually my question was more, does it have good ones on the front and shitty ones on back?

Lines sounds like a great idea too.

Robert,

It has shitty pads on front and rear.. These are all junkyard parts that where not rebuilt prior to install (also before he met me).. he is fighting all the BULLSHIT now.


B

thank you sir, may i have another?

(name that game and win a prize) Andy

What about adjusting the pad clearances to the front disk out, a bit? Wouldn't that require more pedal to engage the brake with a larger clearance? I would think that the rears would then engage earlier as the pedal is pressed?

I have no idea, I am just tossing shit in the air and seeing where it lands!

Just a thought!?

Jeff

pad clearance to the front disc?

hmmm... don't think I know how to do that.
b

My take.....

Repalce the brake lines....then maybe go with a more agressive pad in the rear....then take it and test it.....if the brakes lock in the rear first, go back to the other pads.

As to unequal locking on the front, sounds like caliper rebuild time, inspect the bores, change the pads, check the rotors for warpage.

Last thing to do is an adjustable proportioning valve....more work, but you can dial in the bias.

WTFDIK.......

OK clearance on the rear pads??

Jeff

I don't believe he has stock calipers on the rear anymore, so adjusting venting clearance might be out. Also, an adjustable proportioning valve won't fix the problem as they can only reduce brake pressure, not increase it. You could always build yourself a full blown racing system with separate master cylinders for the fronts and rears with an adjustment on each. I can't imaging that you wouldn't be able to get your brakes set up the way you want with system like that. All it takes is time and money.

Jeff,

I'm sorry.. I did not mean for that to come across as rude what so ever...

seriously, I apologize for the way that sounded..

I don't think andy has an adjustment on his fancy 911 brakes....


brant

An adjustable valve adjusts BIAS...... You can dial in more bias to the rear in a single MC system. In other words, rear brakes can be made to come on before the fronts and/or balanced with the proper slight bias to the fronts....takes some effort but it can be done.

A dual MC gives you two circuits with pressure equal to the stroke and volume of the piston to front or rear.

Not taken that way!! No worries!!

Jeff

I agree with Rice Rabbit.

Sounds like rebuild time. My stock 6 had the same problem with the right front. Here's the pisser Andy; I did new hoses at the same time so I can't say with certainty if it was the rebuild (which I suspect it was) or the hoses. It could have been that the hoses though...

I agree with letting the pads seat as well.

Re the pad thing...I put the grippier pads in the FRONT in order to reduce the rear bias slightly (with a tee not a prop valve)....you have bigger rears and so I figure doing the reverse would be questionable...

It definitely sounds like a weird issue.

Are the rotors in spec and unwarped? (No weird grabbing or pulsing?)

rear rotors were used but look brand new (no wear).
the fronts have been on the car for a while and look used.
the pads are all stock as far as i can tell, the rear pads have more meat left but probably need some break-in time before they grip 100% ...

i'm kinda lost on what to do. sure, i can run shitty pads in the front, but somehow that feels like a band-aid
Andy

Drive it until it needs pads or until you get all the pieces powder coated and ready for install...then budget for a complete brake rebuild.


B