Will a Brake Master Cyl. Designed for Disc/Drum be Adequate for 4-Wheel Disc Brakes? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 04-16-2009, 12:19 AM
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Will a Brake Master Cyl. Designed for Disc/Drum be Adequate for 4-Wheel Disc Brakes?

I am converting my vehicle from power assist to manual brakes. The stock set-up is power/vacuum-assist 4-wheel disc brakes, with a tandem 15/16" bore master cylinder (which appears to be a relatively small bore for a power 4-wheel disc set-up). The original pedal pivot ratio was 3:1, and I have re-drilled the pedal shaft to move the pivot pin up 2" and change the ratio to 5.8:1 (this is the highest the new pivot hole could go without causing interference). I am not changing any other aspect of the braking system. It is my understanding that I should use a master cylinder with a smaller bore when converting to manual brakes, so I have been considering either a 13/16" or 7/8" bore master cylinder, of which I have found several options. All of them, however, were designed for cars with vehicles with front discs/rear drums. I cannot find a manual/non-power master cylinder with a bore this small that was designed for four wheel discs. My question is this:

Are brake master cylinders that are designed for vehicles with disc/drum setups internally different from those designed for vehicles with 4-wheel disc brakes? I know that vehicles with front discs and rear drums will incorporate a proportioning valve for the rear drums, but I am unclear as to whether or not this valve is sometimes designed right into the master cylinder itself (and if so, how do I determine if a given master cylinder has a built in proportioning valve?)

Additionally, is the rear bore of a tandem master cylinder a different size than the front bore, and if so, is it based on a ratio of the size of the front bore (and is this ratio different for master cylinders designed for disc/drum vs disc/disc)?

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Old 04-16-2009, 12:46 AM
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If I were you, and if your brakes worked well with the booster, I'd try the master cylinder you have already. If, with the pedal ratio change you made, the brakes are still too hard, you can think about changing the master.
I know that I did a Baer brake swap, 4 wheel disc and the recomended MC bore size was 15/16, power or manual. Those front calipers had 1 5/8" 2 piston fronts and single piston rears... (can't r'member the rear piston size right now...doh!)

As the bores get smaller than 1" , the pressure differences between the next lower size becomes less noticable. Be aware, some of the smaller bores also have shorter strokes, you want to keep as much stroke as you can. Most dual master cylinders have about 1.1" of stroke. Any less than that and you might wind up running out of the extra volume you need at the piston in the event of a system failure on one end of the car.

You can use a disc/drum MC on a 4 wheel disc the differences are:
1) the disc reservoir size is larger than a drum brake reservoir, the only thing you need to do is check your brake fluid in the rear reservoir more often, as it is the volume of fluid in that reservoir that "adjusts" the position of the pistons to compensate for pad wear.
2) there is an internal residual pressure valve built into the port that goes to the drum brakes..This is easily removed, using a screw and washer to pull the inverted flare brass insert out of the port, then removing the rubber valve and spring, then replacing the brass insert.

It's easy to check for the RP valve's presence, you just poke a small wire into the port, if you feel something soft about 1/2" in, it has the valve...if you can push the wire in about an inch or so, it has no valve.

There is no proportioning valve in 99.9 % of the master cylinders you would commonly find . The MCs that I have seen that have them have a large extra casting on the side, with the brake ports there. They are expensive too.

There are step bore master cylinders, but typically those have bores in the 1 1/8" and up range. Those are used in some of the later model GM cars with the low drag calipers. The large bore is for the front calipers, the smaller bore for the rears.

Internally (piston, plunger,springs, etc) I have not found any differences between disc/disc and disc/drum setups.

The guys at Power Brake Exchange in San Jose could tell you for sure, about a specific MC, those guys rebuild master cylinders and all kinds of brake hydraulics on site, and are a wealth of that kind of info.

Hey Cupertino Howdy from Gilroy.


Hope this helps,
later, mikey
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Last edited by powerrodsmike; 04-16-2009 at 12:55 AM.
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Old 04-16-2009, 03:03 AM
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Thanks for all the info Mikey. One more quick questionóDo you know if master cylinders used on front wheel drive that use a "split diagonal" brake plumbing setup are internally different than RWD cars? Or is the difference, i.e. the split diagonal aspect of the braking system, strictly a matter of different plumbing, external of the master cylinder itself?
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Old 04-16-2009, 09:38 AM
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Quote:
Originally Posted by mrwumper
Thanks for all the info Mikey. One more quick questionóDo you know if master cylinders used on front wheel drive that use a "split diagonal" brake plumbing setup are internally different than RWD cars? Or is the difference, i.e. the split diagonal aspect of the braking system, strictly a matter of different plumbing, external of the master cylinder itself?
I would think they are the same, and it's all in the plumbing, but I wouldn't bet my life on them being exactly the same.

I know that regular RWD tandem master cylinders have 2 return springs in the bore, one is stronger than the other, what the exact reason is for that, I couldn't tell you. I always thought that it had to do something with the port closure timing, but again, that's a guess.

Later, mikey
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