Brake Line Pressure

Been looking for the specifications for brake line pressure for the Porsche systems. My M/Cs are not sized right and I want to measure the pressure of the two master cylinders in my dual setup. Anybody know what the pressure range is? Thanks!

Can't help you there. What size MC's and brake calipers are you using?

Big Blacks up front, 944Turbo/S2/968 fronts on the rears. M/C size recommendation was 3/4 inch front, 1 inch rear. Pedal is very firm, but the car does not stop well, wouldn't take it on the road! I played with front and rear bias, and today I swapped M/Cs, re-bled, not much difference. At 80-90 bucks a pop for M/Cs, I don't want to go through a bunch of M/Cs to get it right. I have a brake pressure gauge, so wanted to see what the pressures are, and which way I need to go. Smaller is higher pressure, so, I guess that will be the next step.

I have an extra .70 inch if you want to try it out PM your address.

Found these on another site from someone that has installed a dual master setup:

I have a tilton dual master setup and am EXTREMELY HAPPY with it. the bore sizes I was using was 3/4' front and 5/8' rear with big reds in front and stock front calipers in the rear. I have a balance bar inside the car. I must stress that this is a street car that I do 5-10 track days a year in, and I have no problems with pedal effort at all.The difference in brake feel and ability to modulate threshold braking is every bit as communicative as the skip barber formula cars in which I normally race(and were the inspiration for the removal of the booster.) This upgrade was very reasonable in cost (<1000) and I
REALLY like it.

and another about a year later by the same person

I have a tilton dual master cylinder setup that was mounted to the firewall with a bracket welded up from flat bar stock. The adjuster is just a threaded rod that varies the amount of pressure applied to each master cylinder by changing the distance of the brake rod from the center of the balance bar, a really simplistic but very effective mechanism . I'm using 7/8" front and 3/4" rear bore sizes with big reds front and '86 front calipers in the rear. I spent less than 700 on this setup and as far as bias goes, you brake hard in a straight line at threshold and keep moving the bias to the rear until you reach a point where the rears just start to lock up first... and then move the bias back to the front a little. This may change depending on track conditions, how much trail braking you have to do, etc... I really love this setup and wouldn't go back to using a booster/stock arrangement, at least on this DE car


I think the stock front calipers that he put on the rear were are 86 turbo front brakes.

Thanks Arthropraxis! I will PM address. What size M/Cs and calipers are you running?

I have stock 87 turbo brakes with a .70 front and 13/16 rear. The Tilton rep. specified 5/8 front and 13/16 rear. I bought a Tilton dual MC mount off ebay with two .70 MC installed, so I figured I would try one of them and see how it works out. They are fully rebuildable, if they leak it is an easy fix and less than new.
The quotes I posted were interesting. He runs the smaller MC on the rear and the larger on the front. The first picture in the dual MC thread with the LT runnin dual MC side by side did the same thing. I remember reading his thread. He started with the larger MC in the rear but could not activate his ABS. When he switched them he had much better braking.
Maybe, the .75 does not have enough volume for the big blacks.

Can you easily lock your brakes up? How is your bias setup? Thanks,

I have only driven mine 10ft so far. At this point I am just going by the recommendations by the Tilton Tech.
Here is a good article on MC sizing and has a calculator on it that you can input data to. The calculator will indicatie if your line pressure is sufficient.
http://www.pirate4x4.com/tech/billavista/Brakes/#MasterCylinders

Mike,

When you say big blacks, do you mean Turbo S front brake calipers?

I have Turbo S calipers in front and 968 front calipers in the back. I believe that both master cylinders are 3/4. For my setup, using the 968 fronts in the back is too much rear brake. Even with the ability to regulate pressure with the DMC and an in cabin pressure regulator, I have to run street pads in the rear to keep from locking up. I will be installing 968 rear calipers on the rear of the car as soon as my set are rebuilt.

Hi Lemming,

Big blacks are from the 928GTS, same piston size as big reds, black, not sure what the 944 Turbo S had, but I wouldn't be surprised if they are the same. Unfortunately, I sold my rear 968 calipers, I'm thinking of looking for a set again. It looked to me that the front calipers didn't cover as much rotor on the rear as the rear caliper did.

I'm beginning to think there is a fundemental problem other than M/C size, possibly pedal throw or I am just not pressing hard enough on the brakes. If I press any harder the car would be useless to drive!

What brand M/Cs are you using? What is the pedal feel like? I moved my pivot point up 0.90 inches from stock, which should give me a pedal ratio of 6.65:1, according to Wilwood's chart, I should see about 1500 PSI with the 3/4 inch master.

The only other thing about my setup is I am using - 3AN braided lines from the M/C to the T at the firewall, and braided lines at the calipers.

As soon as adapters come in for my brake pressure gauge I can measure pressure at the calipers and see if that sheds any light on the subject.

My pedal is hard! I'm getting more and more used to it, but I still consider it a problem for really high speed deceleration (like T5 at Barber and T10a at Road Atlanta).

I'm using AP racing M/C's.

I had to install a Tilton adjustable proportioning valve to back off rear brake pressure. I have factory Turbo S brakes from and rear and a Ford Mustang hydroboost master cylinder. On the street it was fine, on the track too much rear brake bias.

Got this info from another site, I personally have not done the math to verify the numbers but it "looked" about right.

6:1 pedal ratio
100 pounds of applied foot pressure
3/4 1359
13/16 1158
7/8 998
15/16 870
1 764
1 1/8 603

*** Proportioning valves for the rear brakes should not be needed with dual master cylinders. Balance should be "easily achieved" with correctly size master cylinders and then fine tuned with the balance bar.***

That said, I'm going to make an effort to round up a set of pressure gauges and check my actual pressures at the calipers in the next few weeks.
The master cylinder on my daily driver has recently started leaking a little at the booster. I'm thinking I'll go ahead and convert it to manual now, maybe I can have my brakes figured out before the engine is ever removed.

Mike, don't you have a bias adjusment on your mount and another in your car? Maybe that is doing something with your setup. Just a thought.

Thanks, I appreciate the feedback. Yes, I have both, I have the bias valve currently set at full brake. Assuming with it set like this, I am working with only the bias bar. I have played with the bias like this, starting with an approximate 70/30 front to rear, and many variations including a stragiht 50/50 split and using the valve to reduce the rear brakes. At one point I just ran the front brakes, and they were pretty weak, and these are speeds less than 30 MPH. Not sure what is up, I can certainly take the valve out of the equation, spending a small fortune bleeding brake!

reclaimer wrote:
*** Proportioning valves for the rear brakes should not be needed with dual master cylinders. Balance should be "easily achieved" with correctly size master cylinders and then fine tuned with the balance bar.***

Although I agree with the premise that the balance of the brakes should be achieved with the M/C's and balance bar, having a rear brake proportioning valve in a race car is critical. I change my brake balance based on fuel load and track conditions.

Lemming, I understand completely, I was just passing along the info I had read.

I borrowed some gauges this afternoon. Actually got hold of some quicker than I thought I would. One set goes to 1000 psi and the other set goes to 2000 psi, unfortunately neither set has bubble flare adapters in them so I'll stop by a local parts store tomorrow and get a piece of brake line.
I don't know if it will be of any use to anyone, but I'll post my results. If nothing else you will at least know what the stock vacuum boosted master cylinder pressures are and the difference between front and rear. It might help you figure something out.

That would be a help. If you had a way to measure the force at the pedal it would be even better.

Reclaimer that would be very valuable information to me for my troubleshooting. Thanks! My adapters for my gauge should show up tomorrow, I'll measure and post what I find as well. If I can find the right gauge I'd like to know how much force I am putting on the pedal. Although, based on the Wilwood chart, I can calculate the pressure on the pedal. I'm also going to measure how much throw there is at the M/Cs, to make sure that is not an issue.

One bit of info from earlier in this thread. The 928 GTS "Big Black" front caliper has the same dimensions as the 993 twin turbo "Big Red", just switched for trailing strut. The 944 Turbo S front caliper is slightly smaller and is the same as the 928 S4 and the 993 non-turbo. All of these front calipers have the same piston sizes, 36mm/44mm.

The 944 Turbo calipers have pistons sizes of front 36mm/40mm and rear 28mm/30mm.

Rich

I used the quick and dirty method for these numbers but these are maximum pressures.
Bone stock 83 model. Specs posted indicate stepped bores of 19.05mm and 23.81mm. I have not disassembled one to verify these sizes. Also, there are no portioning valves on early models either.

I dead headed the master cylinder by mounting two gauges directly on the output ports. One for each circuit and plugged the second outlet of the front circuit.
I used a six inch section of steel brake line for each gauge and then had the guy I work with assist me in bleeding the gauges to remove all air. The only air that could have possibly been trapped was a BB sized pocket at the transducer inlet.

Engine running, full vacuum assist.
Front, 750 psi. Might have been closer to 748, but the width of the needle on the gauge was 5 psi.
Rear, 675 psi. Might have been closer to 677, but again, trying to read the tiny little lines was troublesome.

The 750/675 numbers are what I'm using for reference as base line stock. I didn't swap gauges and retest to determine the average but I'm comfortable with these gauges being really close to the same. They're the same brand and model and show no signs of abuse.

Keep in mind that these pressures are dead head numbers at the master cylinder. These are the absolute maximum and my car (daily driver) never ever sees these pressures in normal operation.
For actual operating pressure (what the caliper gets) the gauges need to be screwed into the caliper bleed screw location.

Hope that helps, sorry I don't currently have access to dedicated test adapters to read pressure at the calipers. But keep in mind that pressure at the caliper has a ton of variables.

Hold the phone.... One of those pressure numbers is wrong. I had both circuits in a dead head condition at the same time. One cylinder had to lock before the other.
I've already returned the test equipment, but I'll borrow it again Monday and retest each circuit individually.
Sorry for the false start there, but at least one of those numbers should be correct.

I'm embarrassed, but the original pressure numbers are correct after all. I retested each circuit separately this morning and got the same pressures as before.
The front and rear circuits do not share a common or one piece shaft assembly. There's a heavy spring between the stepped sections of the master cylinder that isolates and allows two cylinders of different diameters to operate together.

Test your line pressure, if you've got at least 675 psi on the rear and 750 psi on the front... that's all there ever was to start with on a vacuum boosted early model.

Just one piston seizing will cause hard pedal pressure. When used calipers set for a long time, the pistons can seize easily.

Also if you mis-match caliper piston sizes (86T front calp's vs 88T front calps) 36-38mm vs 36-40mm for the area calculations, pedal pressure will increase.

That is good information. Mike, did you do your pressure with the dual masters? it would be interesting to see what the numbers are.