how did you figure out which or what size master cylinder to run on you ,I have volvo disc on the front and caddy disc on the rear,car had disc /drum before
I thought I heard someone calling my name! I have Wilwood disc brakes on my roadster front and rear with a Wilwood adjustable proportioning valve. This master cylinder is Corvette, I thought it had a one inch bore but it actually measures 1&1/16" with a dial caliper. Not sure where I got it but I do recall my chassis builder initially supplying a different one with ports on opposite side, more than likely swapped it for ease of plumbing.
If your brakes are boosted you can use a larger diameter master, if not boosted use a smaller diameter. I use a 7/8" master cylinder on my roadster, non-boosted with a 7 to 1 pedal ratio. This gives a very light brake pedal feel on my lightweight car with 4WDB. A 1" master would also work but would give a slightly higher pedal pressure. Pedal ratio is important on non-boosted systems as you must have adequate leverage. 6 or 7 to 1 is good for non-boosted systems. https://techtalk.mpbrakes.com/how-to-series/correctly-calculating-pedal-ratio You'll be using an adjustable proportioning valve to delay/dial out most of the pressure going to the rear brakes so you don't really have to consider the rear brakes very much at all. They are pretty much "just along for the ride" and to balance the braking effort and "settle the chassis" during braking. Almost all the work is being done by the front brakes. Adjust the proportioning valve starting with it wide open, then close it until the car "squats" equally front and rear when the brakes are applied BUT THE REAR WHEELS DO NOT LOCK UP. Be sure to do a few trial stops on wet pavement to be sure the rear brakes don't lock up under hard braking when the road is damp - this is never pretty! As I've discovered it's not rocket science as long as you don't try to cut corners or use re-purposed components which are not compatible with each other.
I always use the master cylinder that came stock with the front calipers I'm using. Never had a problem by doing this.
Within reason it really doesn't matter what they are or where they came from. The master cylinder doesn't care as long as you are using residual pressure valves front and rear - 2 lbs for discs, 10 lbs for drums. In your case you'll be using 2 lb residual pressure valves to the front and to the rear. Disc brakes need a little pressure - 2 lbs or less - to keep the caliper cups from receding regardless of where the master is located. They are designed so that the pads drag slightly on the rotors all the time. Only a small amount of fluid actually moves when the brakes are applied to increase the pressure of the pads on the rotors and slow the car. Drum brakes operate on a different principal altogether which is why they require 10 pounds of residual pressure instead of just 2.
Cool. I wish the guy I got my latest resurrection project from did. He used a single-circuit drum brake master cylinder with a built-in residual pressure valve on a 4WDB brake system...and drove it that way.
I was looking for information on what bore and do I need a proportioning valve Sent from my SM-J737T using The H.A.M.B. mobile app
Since there aren't a lot of people chiming in here I guess you're stuck with me for the moment. Do you still have questions I can help you with?
My questions are still the same bore size on the master cylinder and do I need a proportioning valve in the back Sent from my SM-J737T using The H.A.M.B. mobile app
Okay, glad to help. You haven't said but your avitar is a Model A so I guess that's what you're working on. You are going to run without a booster so a 7/8" or 1" bore master will work just fine. Pete & Jake's sells them in the popular Mustang configuration for under-floor or firewall placement. Use a 7:1 pedal ratio which is what most popular under-floor hot rod brake pedal kits have. The dual-circuit master cylinders like the Mustang-style will use two residual pressure valves. In your 4-wheel disc brae system you'll have a 2 lb valve to the front discs and another 2 lb valve in the rear disc brake circuit. I like the Wilwood in-line units myself. You will absolutely need a manual in-line proportioning valve in the rear brake circuit. It goes between the rear residual pressure valve and the brake calipers. Again, Wilwood makes a nice one that's inexpensive and high quality. I went over the proportioning valve adjustment procedure previously: start wide open and close until the car "squats" comfortably on high braking effort but does not lock the rear tires. This is done by trial and error in a safe area. The new cupro-nickel brake tubing makes fabrication of your system much easier than the old style rigid steel lines.
Not to step on anyone's shoes here....but a proportioning valve will not "dial pressure" front or rear. The ONLY function of a proportioning valve is to define the "cut-in" point of the rear brake circuit. I,E: the point at which the rear brake line is opened to pedal pressure. Whether or not you need a proportioning valve depends on a number of factors. If you have the common front disc/rear drum setup on your ride, you likely need one. I put rear discs on one of my cars and found that I no longer needed a prop valve with the massive tires I am running out back. Yes...tire and rim choices DO alter braking characteristics. Drum brakes are self energizing and therefore tend to lock up much sooner. Take the car out and perform a few panic stops. The odds are that the brakes on at least one end of the car (i,e: front or rear) are going to lock up under panic stop conditions. What you need to make certain of is that it is the front brakes that lock first. If your rear brakes lock first your car MUST change direction in a panic stop. The prop valve allows you to set the cut-in point of the rear brake circuit, and therefore prevent your car from swapping ends as you're trying to skid that car to a stop.
The only time disc brakes need a 2 lb residual is when the calipers are higher than the master cylinder, to prevent fluid drain to the master. The 2 lb residual is an aftermarket item; modern factory disc brake vehicles all have high mounted master cylinders and do not need a residual. The caliper seal is designed to slightly return the piston during brake release.
A proportioning valve allows full master cylinder pressure to the rears until the "cut in" or "crack point" is reached, then starts delivering a percentage of increasing master cylinder pressure to the rears.
A proportioning valve does not open to allow pressure to the rear, a proportioning valve limits pressure rise to the rear once it hits the split point. Sent from my iPhone using H.A.M.B.
Along with the lathe cut seal in the caliper there is always a slight amount of runout in the rotor which helps return the piston Sent from my iPhone using H.A.M.B.
SD All three of my project cars have four wheel discs, my feeling is if you are plumbing a new system just put an adjustable proportionining valve in it and be done with it, what can it hurt, then if adjustments are needed you're done, no need to mess with more plumbing.
Thanks for the additional info. Everything I mentioned before is appropriate to your build. BTW: There is no harm done by installing a 2 lb. residual pressure valve(s) in disc brake circuits even if the master is above the calipers. It's a little inexpensive extra insurance that you won't run into problems later. Better to have one and not need it, than to need one and not have it. Same goes for the in-line adjustable proportioning valve. If you build a complete, well-designed brake system in the first place you'll have all the adjustability you'll need to dial it in to function at its safest and best.
I'm curious what problems could result by not installing a 2 lb residual in a disc system with a high mounted master cylinder? No production disc brake system has ever used or needed a residual pressure valve because high (firewall) master cylinder location became the norm in the '50s, years before disc brake usage. Keep in mind using the 2 lb rpv actually works against normal caliper seal retraction and is why you don't want to use one unless it's really needed, and that is only to prevent fluid drain back when the calipers are higher than the master. Drum brake residuals perform a similar function, but for an entirely different reason; that is to keep a low pressure in the wheel cylinders to prevent air from entering past the cups during fast brake release.
What OEM has done over the years is pretty much irrelevant to hot rodders who are trying to combine mismatched - sometimes horribly mismatched - parts into a safe and efficient brake system. The recommendations I make are for a simple hot rod brake system that is tried and proven, and will work and be safe in virtually every conventional hot rod application I can think of. If the OP takes my advice I think he'll be satisfied with the result. If not, that's his decision and he is free to do as he pleases. My goal is to help him solve his problem regardless of his level of expertise. I hope my posts are successful in doing that.
My desire is to help the OP, Bob, not get into a pissing match with you. He can take my advice or not, that's his choice.
missysdad1, I have a prop valve question and you seem quite knowledgeable. I'm pretty new to forums so not sure how to ask you directly, or if it's OK to "hijack" this thread, or what. I bought a '55 "FrankenSedan" with a hodge-podge build from various 60's 70's and 80's Fords, trying to sort out the combination (prop) valve. Is it OK to continue here, or should I start a new thread?
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