master cylinder front - rear

[dfink]

Some issues with brakes locking up on the front of the race car. I have had the car apart so many times that I am questioning if I got the lines hooked back up correctly to the master cylinder. Which connection on the MC goes to the front brakes and which goes to the back. Also which line is supposed to have the factory proportioning valve.

Just about everything I find assumes you just put the line back in where you took it out. I can't find one to just say what is front and what is rear.

This is on a 1984 911 with power brakes.

[Bill Verburg]

Originally Posted by dfink

Some issues with brakes locking up on the front of the race car. I have had the car apart so many times that I am questioning if I got the lines hooked back up correctly to the master cylinder. Which connection on the MC goes to the front brakes and which goes to the back. Also which line is supposed to have the factory proportioning valve.

Just about everything I find assumes you just put the line back in where you took it out. I can't find one to just say what is front and what is rear.

This is on a 1984 911 with power brakes.

The m/c lines are set up as one would expect, the front goes to the front brakes and the rear to the rear brakes

The p/v is on the line leading to the rear brakes

[dfink]

Ok but the master cylinder with power sits backwards in the car. Current the front is connected to rear outlet which is the one at the outside of the master cylinder. The rear is connected to the one next to the booster. Is this then incorrect.

[proffighter]

Bill is right. There are two circuits, 18mm and 14mm stroke. The one with bigger stroke is for front and that's the swimming one, the push rod side is for rear. You can see it here too:

[Bill Verburg]

front of car is to the left, #12 is the p/v, there are 3 different systems shown, which is a little confusing, line 8 is for the front brakes, line 10 is for n/a aspirated n/b rear brakes, the front of m/c is to the back of the car, from the front m/c port(toward the back of the car) line 8 goes to the front brakes. From the rear m/c port line 10 goes to the p/v(12) then to the rear brakes

[proffighter]

Bill

On your first post you said the opposite than now

the drawing of my PET says different between '83 and '84...

I suggest one is wrong, in my eyes one drawing must be faulty, because it makes no sense to switch front with rear while MC is still the same...

I have some cut drawings of the MCV at home which I received once from ATE, I will checked them and let you know. I just remember I had the same question a few years ago...

[proffighter]

Now here is the drawing from ATE. You can see the circuit with the pushrod has 14mm stroke while the swimming one has 18mm. So no doubt the bigger one is for the front calipers. This is from a 930 MC, but don't think it's different for SC's as the lines are the same in PET

[Bill Verburg]

Originally Posted by proffighter

Bill

On your first post you said the opposite than now and the drawing of my PET...

Now I'm confused, on non boosted 911 and on SC the front certainly is the port closest to the actuating rod end and that makes sense from an engineering perspective but the PET diagram for the 3.2 Carrera shows the rear port closest to the actuating end

here's the best pic I have, the p/v is the cylinder at the center of the pic, follow that line up to the port closest to the vac can but there is a hidden section in the middle so it's hard to say



I don't have a 3.2 Carrera here, perhaps someone could take a better pic

[Quicksilver]

From the drawing the front circuit is obviously the one on the left because of the stroke capacity.

From a "lockup the front's" point of view it wouldn't really make a difference if the lines were switched unless there is a horrendous bleeding issue that would allow the pistons to come in contact. Until that point the pressure is identical in both systems. If the proportioning valve was on the fronts it would tend to make the rears lock and the front modulation would be horrible.

Are the front and rear pads different compounds? Any non stock calipers? Are the flexible lines known to be in good condition? Is it possible the proportioning valve is blocked?

[dfink]

Well mine is pretty much like the picture Bill posted. The line cross over at hidden point in the picture. So the front brakes go to the rear port and the rear brakes to the front port closest to the booster. In that case it looks like I have that correct. I would have had to really bend the lines to do it any other way but stranger things have happened.
As to a faulty proportioning valve I took out the original and installed a tilton. It didn't help as all it can do reduce pressure. It can't add pressure so the front still wanted to lock. Really never even mattered how I adjusted it as the front always still locked. I took it back out and went back to the stock setup. One thing I did not try was putting the proportioning valve on the front line as everyone has advised against this. However:
I may try this with caution because the front of the car is so light at only 750# The rear is at 1430# so a 65-35 split which is a bit higher than a normal car. The rear may need more bias under heavy braking. The tilton unit will keep the stock pressure up to a point of 650# adjustable then goes to a 3-1 ratio. I wish I could decrease that ratio. So under heavy braking such as occurs in autocross the front should come on enough to transfer weight then back off to allow all the brakes to work. It is generally the right front that locks. Tilton even shows one of the modes of operation on the right front wheel only. That could maybe make for some interesting stopping. But I will tell you stopping right now is interesting in itself.

[Quicksilver]

With the weight distribution you have I would try it without any proportioning valve.

Also has the car been weight balanced? If the left front is heavy then the right would lock up.

[Quicksilver]

Originally Posted by dfink

. . .
Tilton even shows one of the modes of operation on the right front wheel only. That could maybe make for some interesting stopping. But I will tell you stopping right now is interesting in itself.

BTW - The proportioning valve only on the right front would be for circle track only. They only run left so the inside tire tends to lock.

[Bill Verburg]

Originally Posted by dfink

Well mine is pretty much like the picture Bill posted. The line cross over at hidden point in the picture. So the front brakes go to the rear port and the rear brakes to the front port closest to the booster. In that case it looks like I have that correct. I would have had to really bend the lines to do it any other way but stranger things have happened.
As to a faulty proportioning valve I took out the original and installed a tilton. It didn't help as all it can do reduce pressure. It can't add pressure so the front still wanted to lock. Really never even mattered how I adjusted it as the front always still locked. I took it back out and went back to the stock setup. One thing I did not try was putting the proportioning valve on the front line as everyone has advised against this. However:
I may try this with caution because the front of the car is so light at only 750# The rear is at 1430# so a 65-35 split which is a bit higher than a normal car. The rear may need more bias under heavy braking. The tilton unit will keep the stock pressure up to a point of 650# adjustable then goes to a 3-1 ratio. I wish I could decrease that ratio. So under heavy braking such as occurs in autocross the front should come on enough to transfer weight then back off to allow all the brakes to work. It is generally the right front that locks. Tilton even shows one of the modes of operation on the right front wheel only. That could maybe make for some interesting stopping. But I will tell you stopping right now is interesting in itself.

There is something else wrong

either stock or Tilton on the rear should work out well for you

if the fronts lock easily then look to be sure that the rears are functioning correctly, all 4 pistons most move smoothly, the rear rubber lines can collapse internally and prevent fluid movement too

be sure that the rear pads and rotors are uncontaminated and unglazed

are you using the same compound pads f/r?

[dfink]

Brake lines are only a couple years old and are stainless braid lines all four wheels. The rear brakes seem to work OK as far as grab and release when on a lift so I think the pistons move OK. I know I can force them back if needed. The corner balance is within a few lbs on the front. I have my own scales and check fairly often. I try to balance the front wheel loads for this specific reason. I put on all new rotors last year they are still in excellent shape. The pads are the axxis ultimate on all corners. I have about 1.5* rake and have kept the front torsion bars at 21mm in an effort to promote some weight transfer to the front. The rear bars are 30mm and I have the ASP rear suspension from elephant and polybronze front bushings. The front sway is a weltmeister through body and moves freely.

The front shocks are koni yellow sport adjustables and I generally have them set about 50%. The left rear is the heaviest as the weight of having the driver on the left has to go somewhere. Is there some dynamic under braking that would make the left rear squat thus lifting the right front. Pulling at straws now. My co-driver also has the same front lockup issues though maybe not as bad as me as he is not quite as aggressive.

I just took the car from a wide body to a narrow body hoping the change in contact patch would assist some with braking to no avail. The braking is frustrating because you can't attack a corner if you can't stop. I find I let off early brake light and kind of glide through corners else if I try to go in deep the brakes just lock up and I push off into never never land. Actually very scary and dangerous never never land.

[dfink]

As a side note I have also checked all the bends on all the lines to make sure none were kinked some place. I will try to remove the factory rear proportioning valve and see if that help any. I don't think I have tried it with out that in at all.

[chris_seven]

The only real way is to fit an adjustable pedal box and regulate the braking balance to suit your driving style.

A standard system will always tend to lock fronts before rears and this is why there is a pressure valve on the rear. It is always considered dangerous to have cars which lock rears instead of fronts as they will have a bad habit of stepping out.

By being able to bias the braking more to the rear you will effectively reduce the stopping distance and improve lap times.

Putting a PV on the front to stop lock up may improve the balance but is very likely to increase stopping distances and hence increase lap times.

Chucking away the servo would also help with pedal feel and overall control.

You can affect balance by changing the friction characteristics of front and rear pads but this is always a bit hit and miss.

[Quicksilver]

You mention going from "wide body to a narrow body" so the question is what calipers are on that setup. If you can measure the piston diameter(s), number of pistons, and dimensions of the rotor we might have a culprit.
To completely sort that out, while you are on a measuring spree, getting the same info of the front end would positively sort out any component mismatch issues. With that info and Bill's charts we can easily see what is going on there.

[proffighter]

Originally Posted by dfink

...car is so light at only 750# The rear is at 1430# so a 65-35 split which is a bit higher than a normal car

This is almost the 930 turbo ratio, which has no valve

[dfink]

The brakes are stock for an 84 careers non turbo. It was originally a stock cab that I made a wide body. I just welded the fenders back on last month. The car has been even lighter but I keep putting weight back on as the lighter it got the worse the brakes got. The top is removable and is about 58 lbs but is pretty centered weight. I think I will remove the factory PV and see how that works.

[dfink]

For those curious these are all the same car.

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2012