The Stovebolt Site Forums The Shop Area The HiPo Shop Verify Master Cylinder bore?

A friend is willing to give me a great deal on a new, unused, M/C & Booster combo that won't work on his project. He bought it from a vendor at a show and has no idea what the bore size is.
It "looks" like the ones included in the disc conversion kits, and I'm positive I can get it to fit in my truck, however I won't pull the trigger until I know for sure what the bore diameter is.
Is there a way to get the dimension without disassembly?
If I have to tear into it, I could use some direction as not to mess anything up.
And yes, the booster is currently mounted to the master cylinder.

Thanks,
Tadd

Hy Tadd442, the only way that I am aware of requires at least removing the master cylinder from the booster and measuring the bore at the pedal end of the master cylinder, hope that helps.

I was thinking that would be the way as well...is it safe to assume that the diameter at the end is the same throughout?

Hy Tadd442, the true bore could be smaller, but that has not been my experience, hope that helps.

3B is correct. Two bolts and you can see the bore size. You didn’t state your application (drums or disc). As you are aware, the smaller the bore the higher the pressure it will produce. Drums will require more pressure. My biggest issue with stopping has been the booster size and under floor mounts plus big cammed engines. My suggestion is to run the biggest booster you can with 16” Hg+.

Describe your issues with boosters and under floor mounts?
How big of a cam is too big? I've been considering the ones offered at Langdons and Clifford's...I don't recall seeing the spec's though.

Use a vacuum reservoir with a one way check valve and you can get away with a pretty lumpy cam. I like to use a small refrigerant gas cylinder as a vacuum tank if there's enough room to mount it out of sight under the cab or bed, etc. A tank that size will allow 2-3 brake applications even after the engine stalls before power boost is lost.
Jerry

As in the can used to recharge A/C systems that I can get at Walmart/AutoZone?

No, the canister a commercial HVAC shop uses- - - -about a 15-20 lb. capacity cylinder that's about 10" in diameter and 16" tall or so. They're non-refillable and lots of refrigeration shops will give you an empty cylinder. A reservoir like that will also smooth out vacuum windshield wiper operation with the right plumbing setup. For a smaller version, use a piece of 4" PVC pipe a couple of feet long with caps glued to both ends.
Jerry

Ah... I know what you mean now.
If I can't get one of those, would a helium tank for filling balloons work? Or how about an old fire extinguisher tank? It would probably be easier to hide near the frame rails...

Due to the time the valves stay open on the ‘big’ cams, vacuum is low and therefore you can’t get enough of it so the vacuum booster will not operate to its fullest extent.
Putting a big diameter booster under the floor in my experience is ten pounds of booster in a five pound hole’. If you have the room for a large booster fine, I never did. I had to go with a 8” made for a Mustang, if I remember correctly. That didn’t work well so I ended up with one of those ‘pancake’ types.
All in all, I believe my problems boil down to one issue..pedal ratio. Due to using the orginial pedal setup, which was drum/drum and no booster (39year). The long ‘throw’ on the pedal above the swivel pin and the short distance the brake rod moves is my root problem. If you have the same problem then get ready for a lot of pedal movement before the brakes start to work. The adjustment of the rod between the booster and master cylinder will be critical.
I just read Jerry response. He is correct if the vacuum produced by the engine is your problem. I would call the cam manufacturer and ask about the cam you want in you engine and the vacuum you can expect. The canister is a good idea. You know what will/will not fit so build/buy to that spec. Bigger is better.
Good luck in you quest.

Thanks, I appreciate the advice!

The Helium tank would be overkill- - - -those things are designed to hold 2K PSI or so and they're very heavy. PVC pipe is readily available and cheap, and the end caps are domed- - - -the perfect setup for resisting the considerable force atmospheric pressure exerts on a vacuum tank. A 20" vacuum means that every square inch of surface area of a vacuum reservoir has about 10 pounds of crushing force exerted on it- - - -several hundred pounds, total. Connect manifold vacuum to an empty square-sided 1 gallon paint thinner can sometime and watch what happens- - - -the can will be crushed into an unrecognizable hunk of wrinkly metal.

By putting a rubber grommet into one PVC end cap for a brake booster check valve and drilling and tapping the other end cap for a hose nipple, a very efficient vacuum reservoir can be fabricated. Paint it black and it just about disappears into the C channel of a frame rail.
Jerry

I don't mean the tanks similar to tanks used for welding... Maybe you haven't seen them at Walmart, but they remind me of a smaller 20# LP tank, but lighter weight construction. Either way, I'm really liking the PVC idea! What do you think would work size wise? 3", or 4" diameter by 12, maybe 18" long? What about some sort of pressure relief? As in, if I don't use my brakes/wipers in 20 minutes of engine runtime, will I damage the system?

I'd run a 4" pipe about 18" long. No need for any kind of bleeder valve- - - - -the vacuum will go away on its own within a few minutes of engine shutdown, and there's no problem with keeping some vacuum in reserve all the time. If you want to deplete the reservoir for some reason (can't imagine why) just pump the brakes a few times with the engine off.
Jerry

I realize the engine can only generate so much vacuum, but will the vac in the canister continue to increase as the engine is running, and I'm not using it? I guess it should work... it's how boosters themselves work. I'm just curious, I guess.

Is there any reason why I shouldn't put the inlet and outlet right next to each other?

Either way would work, as long as you use a check valve. I prefer to center-drill and thread the cap and install the pipe nipple fitting on one end and machine the other end for the check valve grommet, but I do that kind of stuff on a lathe. It just looks a little more professional that way.
Jerry

Do check valves come with grommets?
I'm familiar with the concept, but have no experience with them. Are there different types?

Take a look at just about any booster on the firewall of a GM car. There's a small 90 degree plastic fitting (check valve) plugged into a rubber grommet. its purpose is to trap vacuum in the booster chamber to give ONE power-assisted brake application in case of a sudden engine stall. Adding a reservoir simply stores vacuum while it's high (above idle RPM) and makes it available when the rumpy cam you installed during your stupid attack is killing the vacuum at idle. The grommets and the check valves are available at any parts store- - - -even the low end places like Auto Zombie. Just drill or machine a hole in the end cap of your home-built PCV vacuum canister, put an output fitting somewhere, plug everything in, and run a line to your booster. This ain't rocket surgery for anyone with a few brain cells to rub together!
Jerry

Seems simple enough.
Thanks!

What's the smallest booster that will get the job done? I'm figuring on disc front and drum rear.

There are too many variables to answer that question with any degree of certainty. Somebody with a good engineering background would need to know a lot more about wheel cylinder and disc caliper diameter, and the diameter and stroke of the master cylinder, available manifold vacuum, and a few other variables before any answer you get would be anything other than a S W A G- - - - -maybe even a W A G.

Of course, a few others who post here will be happy to pull a figure out of their nether regions with no supporting data whatsoever. That advice is worth exactly what it costs you to read it.
Jerry