Need help with master cylinder theory/operation, please.

[D-CEL]

Dugly,

Everybody has given great intel Ill just add a couple of things
The 120-1051s are Dynalite II calipers with 1.75 bore for a .375 rotor with the standard Wilwood two piece bleed screws, not “speed bleeders”
The CA is the MFG date (in thinking mid 90s)
Everybody is on the money about pedal ratio being critically important.
I would say start at 7-1. If you have the travel, a higher P/R will never hurt you.
I am a fan of dual M/C systems too, but the change from OEM – to “race car” M/Cs is rarely simple and will require a fair amount of fabrication and development.

Jason
this was Rev "A"

now I am up to Rev "D"

[YerDugliness]

I thought I would update this thread.

I let the car sit for a while as I thought about what to do....it seemed like a bit much to approach a new pedal setup and because of the way mine was constructed I didn't have much luck getting a pedal ratio measurement, so I decided to take a stab in the dark and just get a "Corvette style" master cylinder and give it a try.

The previous master cylinder had two reservoirs, but one larger than the other. The "Corvette style" master cylinder has two reservoirs, each the same size. I chose a master cylinder that had larger reservoirs than the previous one, if for no other reason than to increase the volume of brake fluid in hopes that might lead to less heating problems. I also chose a master cylinder with a 1/8" smaller piston, a "special order" 7/8" piece. I did this because I had always had trouble stopping the car as quickly as I would have liked and was following the advice on this thread that a smaller piston bore would result in increased "clamping pressure", which I hoped would result in better braking performance.

Once it was installed I had NO pedal travel...none. The pedal rod assembly had an integral washer...and the washer was specifically sized for the 1" bore on the previous master cylinder. Out came the bench grinder and after a while I had 0.05" clearance all round the circumference...on with bench-bleeding the master cylinder, which was quite easy now that the rod could press the piston into the bore

Well, once it was bench-bled, I proceeded to attach the brake lines. That was too difficult with the attachments on the fender side...no room to work from underneath due to the heat shield between the footbox and the headers and no way to bend my arm in the manner it took to get wrenches on the fittings under the fender. Out came the fender-vent louvre assembly...eureka, easy access now!

With the two hard lines attached it was easy to get on with the bleeding operation, passenger's side rear first, inside bleeder before the outside bleeder. On to the driver's side rear, the passenger's side front, and the driver's side front. My girlfriend did the pedal pressing duty...her first time. She was amazed when the pedal went to the floor as I had described it would when I loosened the bleeder valve for the first time. We went through three large bottles of brake fluid...I flushed and flushed until the brake fluid ran clear from each of the eight bleeder valves. The previous brake fluid was almost milky with absorbed water, and once the brakes were all bled it was off for a test drive.

I must admit the results were far more impressive than I had expected. I probably had 5" of brake pedal travel, and always before the car had needed probably 2" of travel before the brakes started to have any effect. The new MC provides full lockup with only about 2" of pedal travel at the most, feels more like only about 1". I had no idea the brakes were that bad when I got the car, it only had around 3,000 miles on it and I had put on 8,000 of my own.

I can see that there will need to be some sort of adjustability for the rear brakes...although not much, for sure. If I lock the brakes up the rears lock up first, as expected, despite the much larger tires. However, I can't imagine needing to lock them up, they stop the car so much better with very little pressure and so much less pedal travel than before, it's almost like having power brakes (a system I would very much have liked to install, but there was too little room for a vacuum booster).

I have yet to overhaul the Wilwood calipers, I wanted to see how much improvement would result from just the master cylinder replacement. The parts to conduct the overhaul process are quite inexpensive, so the next time the weather gets too nasty to drive the car for a weekend that may happen...but for now I plan on driving it for a while and getting use to having some stopping power....also want to make sure there are no leaks anywhere. It is SO MUCH better than before, stops quickly and straight without swerving or pulling!

Thanks to all who helped with suggestions. I really appreciate the advice, particularly the advice to get a cylinder with a smaller diameter piston. If I ever take the dash out (a distinct possibility) I may attempt to install a better pedal assembly, and at that time a dual master cylinder assembly may well be the ticket. For now it is such a drastic improvement that I think it will do fine...I have never tracked the car and I suspect that it may never see any track duty other than perhaps some 1/8 mile drag time for tuning purposes. It really is just a road toy right now.

Cheers, friends, and thanks again!!!

Dugly

[mickmate]

Hey Dugly excellent report and even better results. Thanks for the update. Does your new master cylinder have two pistons the same size in one cylinder? They call that a tandem master cylinder. That would have two line coming out of it for the fronts and the rears.
You talk about the rears locking first, that's not good. In a panic stop at speed it causes the rear to lock up and try to pass the front end. What you want is a tad less braking on the rear to get what's called a good brake balance. In brake systems with dual master cylinders you can simply change a cylinder size by a 1/16th to increase or reduce the braking on one end as required. That may not be practical with your master cylinder set up so if you simply plumb in an adjustable brake bias valve like this on the rear brake line http://www.summitracing.com/parts/sum-g3905/overview/
Your set up is good for this fix as if the fronts were locking up early you would want to change sizes on masters. You do not want to adjust bias or balance with less braking on the front or put a valve in the front line as they do about 70% of the work stopping due to weight transfer and a gaggle of other considerations. Plumb one of those proportioning or bias valves in to the rear line, have the knob pointing in or up out of harms way and have fun playing with your optimum brake balance.

[RallySnake]

Dugly,

Do not wait, buy that proportioning valve as Mickmate suggested and install it in your rear line quickly. Take it from me, going off the road sideways at high speed is not much fun.

You will need a tubing cutter and flaring tool to do the install. It must be a double flare. If you don't have the tools, a good shop can probably do this for you in an hour.

A better alternative might be to buy an exact replacement master cylinder of the original with stepped bores. You could have your old one rebuilt. A lot of engineering went into that design to get proper brake balance.

Test your car by braking hard in a large parking lot (see my original post). This is important. I lived in Houston many years and the traffic there is very frustrating and requires excellent brakes.

RS

[YerDugliness]

I live in the Huntsville area now...but I am familiar with Houston as I own a home southwest of Houston about 30 miles in Rosenberg. My car is AWFUL in the rain, those huge back tires cause the back end to float and brakes that lock up easy will make it even worse. The valve gets ordered tomorrow and soon as it arrives I'll go out to the "rod shop" outside of town and have it installed as you and MickMate have recommended...without it I can see the writing on the wall! He says he has the tools to do the install and I don't want to invest in a double-flare set.....yet, that is!

Cheers!

Dugly