Brake Master Cylinder - Manual vs. Disk Different?

[contactwilson]

Many catalogs I look through have different part numbers for a manual drum (non power assisted) and a disk brake power assisted master cylinder. Is there a difference?

I have a 1972 Bronco with 4 wheel manual drum brakes. I need a new master cylinder. I bought a hydroboost system out of a wrecking yard. It might be awhile until I have time to install it. I will also be converting to disk brakes later.

Can I use a manual master cylinder and later use the same master cylinder for my hydroboost system with disk brakes?

 

[TOFIC]

Yes and NO.
You can use a master from a drum system on discs but you have to modify the master. You are better off to buy a disc/drum master.
TOFIC

 

[contactwilson]

Thanks for the info. Out of curiosity, what are the functional differences between the two (capacity,valving,etc.)?

 

[ransil]

The difference will be the mount spacing on the MC vs the hydro and the push rod depth will be different.

I listed some of the dimensions here:
http://classicbroncos.com/forums/showthread.php?t=131746&highlight=ransil

 

[Bronco73]

Doesn't a disk MC or any part of the MC designed for disk brakes maintain 2psi of line pressure on the disk brake side?

 

[TOFIC]

Doesn't a disk MC or any part of the MC designed for disk brakes maintain 2psi of line pressure on the disk brake side?

Close but no cigar.. The MC maintains the pressure on the drum side. Pressure varies as to make. Also the Disc side uses more fluid to work so the reservoir is bigger. I have seen onhe MC where the size of the reservoirs is the same on the master for disc and drum brakes. the only difference was the residual pressure valve.
TOFIC

 

[Bronco73]

Close but no cigar.. The MC maintains the pressure on the drum side. Pressure varies as to make. Also the Disc side uses more fluid to work so the reservoir is bigger. I have seen onhe MC where the size of the reservoirs is the same on the master for disc and drum brakes. the only difference was the residual pressure valve.
TOFIC

Are you sure? I thought it was on the disk side to keep slight pressure on the caliper so you don't end up with a lot of brake pedal travel when the pads wear down. The rear brakes have adjusters to cure the pedal travel issue but disks have no adjustment which is why it needs the 2psi of residual pressure.

 

[Apogee]

No OE disc applications that I know of use a 2# residual pressure valve. The only time I've seen them used (correctly) has been in applications where the MC is lower than the calipers to keep the fluid from flowing back from the calipers to the MC, something typically only seen on race type applications. Caliper piston retraction is determined by the design of the piston seals and does vary depending on the caliper design, but has nothing to do with RPVs.

The reason the reservoirs for the disc brake circuits of a master cylinder might be larger than drum brake circuits is to compensate for pad wear. As the brake pads wear, the piston extends from the caliper requiring a relatively large volume of fluid to fill the caliper. Wheel cylinders are relatively small and the self-adjusters on the drums compensate for pad wear, so the risk of running low on fluid volume at the MC due to friction wear is low without a leak somewhere. If you run some numbers, you might be surprised how little actual fluid volume is required to actuate discs versus drums.

The reason drum brakes typically use a ~6-25# residual pressure valve is to maintain slight pressure on the cup seals inside the wheel cylinders. When you release the brakes and the wheel cylinder pistons and seals retract, the lip on the seal relaxes and can allow air to be drawn into the system, hence the use of RPVs. Some applications use a device called a "cup expander" which is just a thin metal cup installed between the seal and return spring that applies pressure to the lip of the seal. The residual fluid pressure is enough to keep the seals from burping air but not enough that the shoe return springs can't overcome it to retract the shoes.

HTH,
Tobin

 

[Bronco73]

okay guess I wasn't sure.