Upgrade my master cylinder.

Here is the old one. Yes, it's under the floor boards and where I need to keep it for the new one.

Space really isn't an issue either. This is a custom brake setup. Drums to the rear and GM 2500HD 8600lb calipers in the front (sounds like overkill on a 4000lb truck, but that's what the conversion kit calls for).

The rod the light is hanging from goes the pedal which is off to the right of the pic.

Bonus points if I can find said master cylinder in a junkyard or you have one to sell cheaply.

What's the app? When we converted an old Ford to dual stage braking, we found many companies that sold adapter plates to go to a modern master cylinder.

This is in a 46 GMC 3/4 ton. I don't want it in the engine bay

What's wrong with the common as dirt mopar one?

That master looks like a big bore unit.

Ranger50 wrote: What's wrong with the common as dirt mopar one?

Which one is that?

That's a common master used on INTERNATINAL trucks 67 loadstar 1700 vintage what you need to know is what bore size does your upgrade kit call for? Hunt around on RockAuto for diffrent bore sizes should be under $45.

I'd go with the master that's designed to feed the front calipers you're going to use. Since the rear will be drums, put an adjustable prop in the line to the rears. Looks like the MC is lower than the calipers/wheel cylinders, so you probably want to use a 2psi check valve in each circuit to keep the fluid from draining back to the MC.

from the support page: disc brake master cylinder with an 8' dual diaphragm booster and a 1.125" cylinder bore is recommended.

If you're staying manual brakes I think you're looking for a 1" (stock, should keep the pedal effort correct) bore disc/drum master cylinder, one that puts the lines out the side opposite that most do, answer will probably be something FWD. You'll have to make a bracket and maybe a new pushrod.

EDIT disregard the lines on the wrong side bit, just realized the MC is spaced way off the inside of a C channel frame rail, just about anything should do. AND the FWD bit, due to X split, stupid X split systems.

Grtechguy wrote:

Ranger50 wrote: What's wrong with the common as dirt mopar one?

Which one is that?

80's era 1500 van

Been over to the HAMB yet? Not trying to take anything away from GRM, but that IS the sort of stuff HAMB is involved with... HAMB

Yeah, this is a question that won't get you banned on the HAMB, that they probably have answers for.

http://www.pirate4x4.com/tech/billavista/Brakes/

It's a long but very good read.

Buried in it is this excel calculator... now it is primarily meant for 4 wheel disc calculations, but in a nutshell do what you can for the front numbers wise, and just run big drums in the rear that work with a prop valve to adjust bias with residual check valves to hold a bit of pressure on the rears.

http://www.pirate4x4.com/tech/billavista/Brakes/Brake_System_Design_by_BillaVista.xls

X2 for the HAMB…

I think I'm going with hydroboost on my "41, but I have a ways to go first.

Using XJ suspension so ill have PS.

flatlander937 wrote: http://www.pirate4x4.com/tech/billavista/Brakes/ It's a long but very good read.

This is the second time this weekend that I randomly stumbled into a Billavista treatise. The first was his page on GM alternators, a good read!

But he made an error right off the bat - he claims disk brakes are lighter than drum, which often isn't the case. They also have inferior static friction compared to drums and they can't pack as much braking force in the same footprint, which he conveniently fails to admit

He also makes some other glaring errors regarding the operation of drum brakes, suggesting his bias may be in part due to a simple misunderstanding of how they work. If this were a movie theater, I'd be throwing popcorn at the screen over how utterly wrong his section on 10psi residual pressure valves is.

In reply to Kenny_McCormic:

FWD MCs are going to be 50/50 split ie equal fluid volume per circuit. The volume demand of those front calipers is going to be much greater than the rear drum wheel cylinders, which is why I suggest using the OE MC for those fronts. The '46 probably has enough pedal ratio that the efforts won't be unreasonably high without a booster.

I didn't mention "without a booster" in my previous post, but that was my intention. Manual system. The MC doesn't care.

In reply to AngryCorvair:

Good point, stupid X split systems.

Knurled wrote:

flatlander937 wrote: http://www.pirate4x4.com/tech/billavista/Brakes/ It's a long but very good read.

This is the second time this weekend that I randomly stumbled into a Billavista treatise. The first was his page on GM alternators, a good read! But he made an error right off the bat - he claims disk brakes are lighter than drum, which often isn't the case. They also have inferior static friction compared to drums and they can't pack as much braking force in the same footprint, which he conveniently fails to admit He also makes some other glaring errors regarding the operation of drum brakes, suggesting his bias may be in part due to a simple misunderstanding of how they work. If this were a movie theater, I'd be throwing popcorn at the screen over how utterly wrong his section on 10psi residual pressure valves is.

Well in the context of 4x4 1 ton axles... You drop about 40lbs swapping from drums to discs on a GM 14 bolt. Which is essentially what EVERYONE ran when the article was written. Many still use them for cheap beef rear axles. Nearly the whole 4x4 community uses GM 3/4 ton calipers and discs/hubs to convert since the parts are cheap and easily done with simple flat brackets. My Jeep used these on all 4 corners.

I tell you what... barking 4 40in swampers to a stop makes for some panicking people around you in traffic...

Why would static friction matter? Unless it's a parking brake or you want to make square tires locking them up?

I concur the 10psi valves are probably a bad idea.

AngryCorvair wrote: The '46 probably has enough pedal ratio that the efforts won't be unreasonably high without a booster. I didn't mention "without a booster" in my previous post, but that was my intention. Manual system. The MC doesn't care.

I looked into this a little deeper, it looks like the MC you're taking about (8600lb 94ish 2500, DORMAN M39649) is 1.25" bore on both circuits, vs stock at 1". Wouldn't that nearly double pedal effort (area .79 vs 1.23), after accounting for the self energizing effect you only get in drums?