Manual Master Cylinder

[72Jesse]

Trying to figure out what new master cylinder to get and searching got me lots of info, but nothing specific. I have front disc brakes and not sure exactly what calipers? Just added Tom’s rear disc brakes which I believe are El Dorado calipers? Need to replace the factory manual drum/drum master cylinder with a new manual master cylinder. Appreciate the help.

 

[ntsqd]

Are you determined to stay manual, as opposed to going either hydro-boost (what I'd do) or vacuum boosted?

 

[72Jesse]

Was trying to stay manual right now to save some $$$.

 

[ntsqd]

Do you realize that the manual m/c might physically bolt onto a booster, but is very likely to be an incorrect size in terms of pedal feel? Said differently, the m/c bore size that gives you a firm manual pedal is going to be too small to give you a similar pedal once boosted.

It isn't crucial, but it would be helpful to know which front and rear calipers you have. Can you post pictures of both?

 

[72Jesse]

I have pictures on my phone, but not seeing how to get them posted without a url?

 

[ntsqd]

I use smugmug, but it does require a subscription. There are other hosting sites that don't, but I know nothing about them.

 

[Apogee]

Your master cylinder bore size should be sized based on the caliper piston diameters, so while we can make educated guesses all day long, it would be better to know what we're working with. Different rear disc brake kits using the GM metric D154 calipers will supply either the parking brake calipers with Ø2.03" pistons or the corresponding front calipers with Ø2.50" piston diameters (and no integral parking brake). If we assume Ford front calipers with Ø2.88" pistons (or Chevy fronts with Ø2.94" pistons) and the Cadillac rear calipers in the rear with the Ø2.03" pistons, a Ø1" bore master cylinder with a 6:1 manual brake pedal ratio would be my recommendation. As with any brake installation, manual or otherwise, pad compound and CoF will also make a large difference in how the brakes ultimately perform, pedal effort, modulation, etc.

 

[Sheehanigans]

As mentioned above, it all comes down to basic hydraulics. Since you can't change pedal travel or hinge ratio easily, you need to size the master cylinder piston diameter to the size of the caliper cylinders. Too small a piston and you'll bottom out the pedal and not move enough fluid; too big and you'll need olympic legs to be able to stop it.

I have manual disks up front (chevy kit) and rebuilt drums in the rear. I run a Raybestos MC39469 with the pressure valve removed and the explorer external proportioning valve. This 1" bore feels good on the foot and is easy to modulate with a nice long throw, but can still easily lock up wheels at speed. You *might* need more volume for the rear disks.

I would try your current setup as-is to try for pedal feel, which is subjective. Then you can choose to go hunt for a 15/16" or 1-1/8" depending on your feel. Confirm you don't have one or remove the RPV from the master cylinder outlet.

Most people that don't like manual brakes say so because they are using a too-large master cylinder that was made for a boosted application.

 

[DirtDonk]

I guess another pertinent question would be, have you been driving it this way?
If so, how does the pedal feel?
And enough stopping power and a good firm pedal?

 

[73azbronco]

it would be way easier, cheaper to go vacuum boost with wildwood master and proportioning valve, easy, works great.

 

[nvrstuk]

TON of good info here (oh so typical) Toss another iron in the fire for ya. Little tires, as in dia and weight work MUCH better with manual brakes than 150+ lb beadlocks and 40"s. Inertia, rotating weight, etc is hard to stop. Ask anybody that's gone from a 30" tire to a 40" tire and they will agree that their stock system has a heck of a time doing what it's supposed to do easily.

 

[DirtDonk]

That’s why the manual brakes on mine always worked so good. Stock master and 32 inch tires. Certainly not big by today’s standards.
Even if they were the norm back in the day.
I loved my manual set up with Ford discs and 11 inch drums in the rear.

 

[72Jesse]

Here are pictures of what I have installed. I was using the original drum master with the front disk and rear drums and ok pedal.

 

[DirtDonk]

Is the old master still installed? If so, and if it were me, I would use it initially just to see how the pedal feel is and how the “power “ is. And if the addition of rear calipers makes the pedal travel too far.
If not, then you have your answer as to what master cylinder piston size is appropriate.
And from there, whether or not you need power assist with a larger master or not.
Then just source a large reservoir master cylinder with the same size piston.

 

[72Jesse]

The old master is still installed so I'm going to bleed things and see what it feels like and will report back.

 

[ntsqd]

Here are pictures of what I have installed. I was using the original drum master with the front disk and rear drums and ok pedal.

Looks like GM brakes all the way around. Those front calipers should have a 2-15/16" piston in them. There is a pretty rare version with larger pistons, but they are rare and it is unlikely that you have those.

Brake pedal feel is HIGHLY subjective. What feels good to one person is either way too stiff or way too mushy to someone else. Let's see how the existing m/c feels to you.

I will predict that the Ø1.00" bore M/C will be a mushy pedal with the front GM calipers, but the next easy to find step up, a Ø1-1/16" bore M/C will be a too stiff/hard feeling pedal.
If a Ø1-1/32" bore M/C can be found that works it might be OK depending on the driver.

About the Residual Pressure Valve (RPV) used in drum brake circuits. It is there to keep the shoes close to the drum surface by not allowing the springs to retract them too far. Disc calipers do not need them and the ~10 psi that they leave in the system will cause the brake pads to drag. The RPV is typically located under the brass sealing cone in the bottom of the m/c's outlet port(s). Those cones will pull straight out, and a small spring and plunger will fall out after the cone is removed. The way to get the cones out is to run a tap into the hole in the cone. Sometimes the act of thru tapping that hole is enough to remove the cone, sometimes you'll need to run a machine screw into the hole to get the cone out. I use a screw partly threaded into the cone as a handle to get it started going back after the plunger and spring have been removed, and then as a punch by hitting the head with a hammer to drive it home in the m/c.

 

[Apogee]

If we ignore the fact that you're working on an early Bronco for a minute, you essentially have the same size brake calipers and rotors as a 1979-1981 Firebird (3300#-3900#), but only if we assume that those D154 rear calipers are the ones with the integral parking brakes (I can't see the inside of the rear calipers in your pictures, but I don't see parking brake cables, so I'm a little suspicious they may be front calipers). As such, the Firebird was supplied with a Ø1" bore master cylinder with a 6:1 pedal ratio in manual brake applications, whereas Power brake applications for the F-body were equipped with a Ø1.125" bore MC mated to a dual-Ø9" vacuum booster with a 3.7:1 pedal ratio. FWIW, a change of 1/16" in MC bore diameter from a Ø1" baseline equates to about an 11% difference in Pressure/Volume, so a 1/32" difference (like with the Mopar Ø1.03" bore units) would be about half that.

 

[ntsqd]

I'm looking at total leverage ratio for the front calipers only with a target window of 93:1 to 95:1. Below 93:1 is rarely acceptable by anyone, out to a bit over 100:1 is acceptable by some. Assuming the existing is a Ø1.00" bore the current OLR is 103.6:1, which will be a mushy feeling pedal to most.

 

[nvrstuk]

Not just mushy but the other direction is not moving enough fluid for discs. Disc calipers take more fluid to move the piston so a smaller ID mc diameter will allow the pedal to travel almost to the floor in some cases and then WHAM, you have brakes. Leverage, pedal ratio , so many factors.

Ex.ample: I kept the same stock '74 mc and slapped t-bird calipers on mine back in the mid to late '90's. Brake pedal feel was perfect AFTER you pumped it once to move enough fluid to get the frt disc calipers to work! lol Got really good at a quick half pump then I could lock 'em up. Obviously this wasn't permanent but I learned a LOT about braking systems as a whole back then as many tired press residual valves in line and we had many discussions on BroncoFix as most of us were looking for a disc setup that would work. Next step was 4 wheel discs... needed more fluid movement...

Love what I've got now...