Installing power brakes on the 39 Poncho. Keeping drums all around. Looking for a suitable master cylinder. Will the Dorman 39052 work? Is there truly a difference in master cylinders when it comes to drum/drum, drum/disc, or disc/disc? I know the proportioning valves are different… but… the masters?
Usually the masters for disc brakes have a larger reservoir. The 39052 should work, providing the bore is correct for your wheel cylinder volume.
Some drum master cylinders have a built in residual pressure valve, which might not work with discs. But generally, any disc master cylinder will work with drums.
39052 has a 1” bore. Not sure how that equates to four wheel drum cylinders. I’m thinking good to go. So, would I even need a proportioning valve for drum/drum?
I just use late 60's drum/drum masters for GM cars like Chevelle, Nova, etc. and they always work fine for 4 wheel drum setups.
The Corvette master cyylinder will work and has a 1.125 bore rather than a 1 inch bore that Agels55 said. Master Cylinder | M39052 | Brake Master Cylinder | Dorman Products I'd have to find my 1946 Motor Manual to find the stock bore of a 39 Pontiac master cylinder. you will have to run residual valves with it though. I'm another like 1971BB427 in that I have used the dual master cylinder listed for a 68 Chevy C 10 for a number of swaps. Not near as exotic sounding as having a "Corvette" master cylinder though. It has a 1 inch bore. The specs on it and the specs on the Dorman 39032. The 39052 will push a bit more volume though. The 68 C 10 (and a long list of others) does have the residual valves behind the seats for the brake lines that are fine for Firewall mount but don't hold residual pressure if the rig sets for long lengths of time at a shot. If my 48 sat for a couple of weeks I would have to bleed the brakes but I do not have any valves of any kind in the lines no proportioning valve or combination valve. I put it together before inline residual valves where common place. I don't see an issue with he 39052 as long as you run residual valves and a proportining valve though. That to keep the rear brakes from locking up before the front brakes and causing you to swap ends.
Keep in mind that increasing bore size of the master cylinder will increase pedal pressure required to stop
^And going too small, there's a good chance of running out of pedal travel or a pedal that's very close to the floor.
You should check a few things and report back. That will help us make better suggestions. What is the original master cylinder bore? What is the pedal ratio? What are the bores of the wheel cylinders? Generally, when retaining the original pedals, I will use a master cylinder with the same bore as the original, unless there is a performance issue with that bore size. Disc master cylinders have larger reservoirs. That is not an issue, so long as there is room to fit it in there.
Find a 4 wheel drum boosted car/truck with the same size wheel cylinders (bore) and copy that system. Of course the weight might be different and maybe pedal ratio, but it will get you in the ballpark.
All of this shop talk is of replacement master cyls. As of the last 5 or 6 years I have little to no confidence in new iron replacements. Surely the majority are manufactured in the exotic lands of Asia, but beyond that this newer synthetic blend rubber for the innards doesn't swell up like the days of old when exposed to brake fluid, and that too is synthetic blended stuff. Not the good old stuff that made a paint job tremble at the mere sight of it. It's not paint safe either but you have a chance to wipe it up. My fix has been to disassemble and put a finer finish on the bores. I like to finish em with 400 wet or dry paper on a shadtree bent welding rod in a drill, get a "polished" cross hatch and then work the parts a good couple dozen times before the bench bleed in the new bore wetted with fluid. This also goes for wheel cylinders. The cars I work on have been known to sit for extended periods. Thats when it fails you. Get in 1 day and your brakes are gone cuz it leaked past a piston. I've had 2 fail me, no more. So far so good. Anyone searching brake issues will hopefully find this.
I seem to post this article a couple times a year, whenever someone asks about master cylinders. https://www.hemmings.com/stories/master-cylinders/
My brain hurts. Lots of great info guys and it is much appreciated. So- from the top: I would like to mount a 7” booster to my firewall, attached to an under dash 1955 brake pedal support with original 55 pedal. The support will be modified to reach the 39 Poncho dash. This should give me the exact same pedal height and throw as my 55. The master & wheel cylinders seem to be my dilemma. Per the 39 Pontiac shop manual, the original wheel cylinder bores are 1” in the front and 15/16 in the rear. I do see ads for 1” bore rear cylinders, so I can go that way. As for residential pressure valves; Reading through various articles, it seems : “When the master cylinder is mounted on the firewall, it is usually higher than the calipers. Gravity keeps the fluid from flowing backwards. In this case, a residual valve is not required”. I see the same conclusion in different articles. Not necessary, but no harm in using them. So I’m back( I think) to the master cylinder question. If I run 1” bore slave cylinders all around, do I go with the Dorman (1.125 bore) or go with the 68 C10 master (1” bore)?
The residual pressure valve in drum brakes is there because they have return springs. Usually not needed in a disc brake because the pad does not retract away from the rotor. A drum brake's return springs pull the shoe away from the drum. With no residual pressure in the wheel cylinders, you would have to pump the pedal to get the shoes to the drum before any braking happens. The amount of pressure needed is a balance with the springs to keep the shoe near the drum, but not dragging. Unless I missed it, what size is the original master ? Mike
Oddly, I can’t seem to find any info on the original 39 master cylinder bore. Nothing in the manual and nothing on line.
The purpose of a drum brake residual is to keep air from entering past the wheel cylinder seals during brake release, because of the strong shoe return springs, that are designed to RETURN the shoes completely to their anchors. With a properly adjusted drum brake, there normally is about .015" clearance between each shoe to the drum, so the notion having shoes running "near" the drum borders on "impractical" but is/has been sadly stated by several brake "suppliers" over the years. Drum residuals eventually were replaced with cup expanders by the mid '70s.
No. Not with drum/drum setup. Works fine, and stops great without one. I do use an adjustable prop valve on disc brake/drum setups. I thought you had changed both front and rear brakes to upgraded modern drums? If that's correct, then what master or wheel cylinders it originally had aren't relevant to the discussion.
Nope, no matter how many people want this to be true it never will be…the return springs return the shoes to the anchor every time otherwise there would be no need for adjusting them. Go measure the force required to extend the springs and tell me that 10lbs of pressure will overcome them. The residual valve are to prevent air entrance upon brake release, if the wheel cylinders have cup expanders you don’t need residual valves at all.
Yep, I don't remember when cup expanders started coming in cylinder kits, but they were available aftermarket for cylinders that did not have them when new. Probably because many cylinders were the same size and were available in different cars in either configuration. But I've been guilty of using master kits with residuals built in and new cups, with expanders. Belt and suspenders kit of deal.
Just now starting to build the brake system. I had the drums turned all around and planned on installing new slaves all around. After all this lively discussion, I think 1” bore slaves all around with the Dorman Master should work nicely. I will likely incorporate the 10 psi residual valves just to be on the safe side.
Four residual valves is 8 possible places for fluid to leak, the master cylinder may already have valves in it.
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