disc brake master cylinder question.

[fuzz]

I know the stock answer but here goes anyway. Why can't I use a disc/drum master cylinder on a 4 wheel disc setup? My problem is I bought a speedway aluminum mastercylinder for 4 wheel discs, it won't bleed properly, can't get the air out, it just keeps makin bubbles. So I have a new 910-31425 mastercylinder that bled easyY, NO BUBBLES. I WANT TO USE IT! TIRED OF MESSIN WITH THESE BRAKES!

 

[Old Iron]

You can use it.
The only thing you have to do is keep a better watch on the drum part of the master cylinder as the brakes wear.
The disc brakes will continue to hold more and more of the brake fluid in the calipers. That's why the reservoirs are bigger on the disc/disc master cylinder.

 

[21willys]

A master cyl for drum brakes don't completely release all the pressure like a disc one does. It will continue to build pressure.

 

[fuzz]

A master cyl for drum brakes don't completely release all the pressure like a disc one does. It will continue to build pressure.

I don't understand why that would happen, explain please!.....

I put the iron one on being careful not to lose fluid or allow air into the system.."BAM" I got solid pedal now. Speedway will refund or replace wihichever I want, so that;s cool, think I'll ask for replacment.

 

[zzrodder]

A master cyl for drum brakes don't completely release all the pressure like a disc one does. It will continue to build pressure.

I'm no engineer but I don't understand that one either [S - if it continued to build and hold pressure it would lock up the brakes - drum or disc.
Now a check valve would hold back some pressure - typical in line checks hold 10lb for drums, 2lb (or is it 5lb ?) for discs. OEM proportioning valves may have the check valves built in.

 

[UncleDonnie]

A master cyl for drum brakes don't completely release all the pressure like a disc one does. It will continue to build pressure.

I have picked up on that also. Don't know how or why. I do know the reservoir size and master volume play a roll in safety.

Don

 

[BlueNorther]

drum brake master cylinders have a small residual valve behind the brass seat in the outlet ports to maintain ~20psi in the brake lines to keep the lip seals in the wheel cylinders from leaking while sitting released. the return springs on drum brakes are strong enough to overcome this pressure. but disc brakes ( at least the ones used today ) don't use return springs or lip type seals, they rely on the "give" in the square cut O-rings to provide a tiny amount of clearance to the pads when released.

 

[Old Iron]

drum brake master cylinders have a small residual valve behind the brass seat in the outlet ports to maintain ~20psi in the brake lines to keep the lip seals in the wheel cylinders from leaking while sitting released. the return springs on drum brakes are strong enough to overcome this pressure. but disc brakes ( at least the ones used today ) don't use return springs or lip type seals, they rely on the "give" in the square cut O-rings to provide a tiny amount of clearance to the pads when released.

Thanks BN
Then on drum brakes, it's unnecessary to have a 10 lb. residual valve when using a drum type master cylinder ?

 

[21willys]

drum brake master cylinders have a small residual valve behind the brass seat in the outlet ports to maintain ~20psi in the brake lines to keep the lip seals in the wheel cylinders from leaking while sitting released. the return springs on drum brakes are strong enough to overcome this pressure. but disc brakes ( at least the ones used today ) don't use return springs or lip type seals, they rely on the "give" in the square cut O-rings to provide a tiny amount of clearance to the pads when released.

This is what I was getting at I just worded it horribly. That's what happens when you drive for as long as I do and then post lol.

 

[fuzz]

Thanks for all the replys. I talked with speedway tech again and they tell me that "back in the day" drum brake masters had check valves but their master cylinders, the one I used, do not have a check valve so I won't have the pressure buildup problem. But like Old Iron said I should keep an eye on fluid level in the small resevoir. I haven't been able to road test the A-bucket yet but the pedal sure feels good in the garage. Thanks again, this is the best place to go for questions...