I had exactly the same symptoms as Artic articulated in his first post. Exactly!!! My car was built in 92 also. I drove the car for years with poor brakes. Put in a vacum canister and everything. Finally I discovered that the booster was shot. Put in a new booster and now the car stopps very well even with the little 9" Mustang II disks. I wouldnt want to road race it, but for the street it is fine. One thing that I would like to do is put in some better pads with more bite, but I can't find any aftermarket performance pads for the Mustang II brakes. If anyone knows of any, please let me know.

Ed

You can buy pads for the Mustang II from Porterfield (good stuff). Here is the link:

http://www.porterfield-brakes.com/pads.html

9" rotors! Yites!

I can definiatly buy the booster issue. That could be part of the problem for others.

Rick

For those of our with high and hard peddles but no stopping, test your booster to eleminate it as a cause. Here is a simple generic procedure that I found for testing the booster:

"Hook up the vac guage. Start the engine. Note the reading. This is your "base" reading.

Stop the engine, disconnect the booster vacuum line and cap the port on the manifold. Start the engine, look at the guage. If this reading is substantally higher than the base, you have a leak in the system, probably in the lines leading to the booster.

Stop the engine, remove and test the one-way valve (should be able to blow into it one way but not the other). Air should only pass in the direction indicated by the arrow printed on the top of the valve (make sure to reinstall it in the correct direction).

Reconnect the one way valve and vacuum line to the manifold and start the engine. Watch the vac guage as a friend depresses the brake pedal. It should dip as the pedal is depressed and return to the base reading. If it drops slowly, but returns, you have a blockage/collapse in the vac line. If it falls and does not return, you have a torn diaphram in the booster. Especially indicative of a torn diaphram is a hissing when the pedal is depressed. "


Rick

Common cause of early booster failure:

http://www.carquest.com/techbulletins/reman/42-50.pdf

By the way, most booster where designed for 17-22 inches Hg. Most of us probably are NOT pulling that much!

Rick

Another quick check:

"A quick way to check the vacuum booster is to pump the brake pedal several times with the engine off to bleed off any vacuum that may still be in the unit. Then hold your foot on the pedal and start the engine. If the booster is working, the amount of effort required to hold the pedal should drop and the pedal itself may depress slightly. If nothing happens and the vacuum connections to the booster unit are okay, a new booster is needed (the vacuum hose should be replaced, too)."

Of course if you can create a vacuum source you can hook it up to the booster. If it does not hold vacuum, time for a new one.

And again, anything below 16-18 inches Hg and the booster will do little good. So if you are running a stock 5.0, you are good, if you are running a screamer, you are going to need a vacuum pump if you want to stick with power. Fortuniatly the dual diaphram design of the mustang II booster does help this situation a bit but there is only so much that can be done.

Rick

Please tell me your vacuum reading at idle and whether or not you are having the problem.
Thanks
Rick

Rick, I just called in an order for the brakes. Thanks for the tip, they sound great!

Ed

No problem Ed! Many of the guys swear by porterfield pads. While I have not used them I have heard enough great reviews from people whos opinions I trust to recommend them. What compound did you end up going with?
Rick

I got the R4-S. I just want good bite and friction for the street. I don't race. Only $79 too! Thanks again. Here are their specs:

"R4-S High Performance Street and Autocross
For high performance and heavy-duty street conditions. Perfect for prolonged everyday street use while also being capable of tolerating the most severe street use without any fade. Rotor friendly of course. The R4-S friction level will give you an impressive increase in stopping ability with very minimum pedal effort. R4-S compound has the absolute lowest noise and dust levels, far below OEM equipment or any other high performance brake material. Good for autocrossing, some drivers schools, solo events, and rally’s. The R4-S compound is available for virtually any vehicle sold in the US. We also offer the R4-S in pad sizes for competition type calipers that are used under street driven conditions"

Ed

Those sound perfect! I would love to see a temp versus cf graph on the porterfields but when asked they could not provide one.
Rick

Rick, I really appreciate the effort you are giving us in solving this issue. One thing I never mentioned is the system was designed around the 9" brakes in the front and the drums in the rear. This system seemed to work pretty good. The problem seems to occur when you upgrade to rear disc or bigger front brakes or both. I have tried so many different configurations over the years I have lost count. Thanks.
Don

Ha Ha, . . . My little 9" disks and drum brakes in the rear work petty darn good. Sometimes simpler is better. With the new bolt in performance pads on the way, I should be in really good shape!!!!!!!!

Ed

My pleasure! I really do enjoy this stuff! And if I can help, that would be great.

VERY interesting about the change from the original design and what the results where.

CR owners, please provide some feedback on my prior questions.

I have been playing with booster calculations on and off for the last couple of days. My system is a dual, non boosted type system.

A couple of things I have found, if you have a booster, less than 18 inches of vacuum is NOT going to cut it. There are a variety of electric pumps available and you can even get them out of the junkyard on various GM models (even GM couldn't over come low vacuum in a power system, why should we be able to!).

Just for fun, on a single diaphram 7" booster the different in peddle effort between 18 inches of vacuum and say 12 inches is is about 115 pounds of assist. So lets say your car had 18" of vacuum and all is well, then you change the cam, etc and end up with 12" of vacuum. You will have to press the brake peddle 115 pounds harder to stop the car at the same rate you used to have to push!!!! Try pushing on a scale some time say at 165 pounds (50 pounds before the cam, 165 after), that is a LOT of force.

Double diaphram, still working on it.

Rick

Rick,

But but b b b but, . . . . . . . Think about this. With an aftermarket vacum canister, it holds the MAXIMUM vacum available, not just the vacume at idle like if there is no canister. So, if you are cruising along at 45mph and then take your foot off the gas, you may pull 25" of vacum regardless of what your idle vacum is. It is that 25" that the canister will store!

Ed

Yup, Ed I do beleive a canister would help. But I would also think there would be some volume/timing/repitition type issues. Never really tested one in the real world. I have got to think there is a reason why GM went with a $250 pump over a $25 canister in certain applications. But again, I beleive that a canister will help.

Do you have one????

If you do, give it a test if you would please. Hook up a vacuum gauge to it and the motor with a tee. Start the car and measure intake and canister vacuum. Bring it up to cruise Speed (vacuum under load will be higher so if possible while the car is being driven), write them both down. Let off the gas, write them both down. Apply the brakes and note what happens. release and apply again. I would be VERY curious to see what happens.

THANKS
Rick

Hello!

Can those of you with the problem currently please give some more details?

Does the Classic Roadster contain any stock distribution blocks, proportioning valves, switches etc?

I understand that some of those with the problem have tried new booster with out and success. I also under stand that while bleeding the fronts often seem to have no pressure. What is your experience?

Any one in Ohio with this issue???? I am in Columbus and if you could come to my place, I beleive I will be able to help.

Rick

Rick, thank you for your help and insight into our problem. I have 11" granada rotors with Camero calipers on front and 8.8 rear end with stock drums on back. I have the stock Mustang II master cylinder, brake booster and proportioning valve. My brakes have been poor from day 1 (1992 CR on the road in "95). I found that, if I use the emergency brake in conjunction with regular braking my brakes work much better. This tells me that the rear brakes are not working hard enough. I just bought a Wilwood proportioning valve that I will be installing. I also just received Don Scott's redesighned pedal that I will be installing soon. My problem is that I am just changing engines so the cobra is down right now and I won't be able to test the new parts for a while.

I did try a vacuum test on the booster as you suggested. I found the booster not to be working at all. I could not get the vacuum pump needle to even flutter. I also sucked on the tube and got a gasoline taste. So I will take your advice and install a check valve in a high location.

I will work out these problems yet. Thanks to you and Don for your assistance.

Paul

Paul, this is not the first time I heard about the parking brake helping with braking. For me it just adds to the mystery!

The system for CR that I am working on though is a four wheel disc design so unfortuniatly I will not be able to help much with drums. I do not have the software not computation to determine drum brake torque.

One thing that comes mind on the drum brakes is there are differences in the pieces used. Be sure that the master cylinder and proportioning valve, the stock pieces are for drum brake use. I don't remember which has a residual pressure valve (10 pounds or so) but you need it. Plus I recall something about a delay when activating one or the other in a mixed system. Either way, if both are for a mixed system, then great!

Also, you may be out of adjustment in the rear. Can you get either end to lock up, if so, which end?

PLEASE tell me exactly what you did to test your booster. Very curious. If your booster is not working, that of course would explain a lot. Very high peddle effort with very little results.

I know that gasoline getting in the booster will shorten its life span.

Are you saying that the booster did not have a one way check valve in line to the vacuum source? Also a no no. You can test the valve by blowing through it. Blow in one end and the air should go through, blow in the other and it should not. Any thing else, pick up a new one.

EVERYONE, what size brake lines are you cars plumbed with? Not 1/8" I hope!

Keep it coming guys!

Thanks
Rick

Rick, Here's my setup: Front= Masterpower 12" rotors with dual piston aluminum (corvette) PBR calipers. Rear = 95 Cobra 11.75" rotors and single piston calipers. 3/16" brake lines, adjustable proportioning valve. Mustang II booster, Don's new pedal, and the master cylinder, from MPbrakes, is a GM (corvette?) that they recommended. I previously had a Napa corvette master cylinder that Don had recommended, I changed it when I changed out the 9" front to the 12". I have not had the time to check the booster as you described. I do recall that there didn't seem to be very much pressure at the rear when I bled them, sometime last year. Also, adjusting the proportioning valve fully either way, doesn't seem to make any difference. Two thoughts that I have considered, one, MPbrakes recommends the use of a metering valve, (even with four wheel disc), and two, Trying a different booster and pedal assembly from another car (other than the mustang II). My brakes will stop the car, but they are NOT high performance brakes as you would expect from this setup. Thanks for your time and any knowledge that you can share with us CR owners that want HIGH PERFOMANCE BRAKES.

Shane

Quick question please, do you happen to know the piston sizes?

Metering valve, interesting. I will have to see why.

Thanks
Rick