Disk Brake Master Cylinder To Drum Brakes

[Super Jamie]

I understand drum brake master cylinders have a "residual pressure valve" to keep say 10psi of pressure in the brake lines, as drum brakes require this to compensate for spring return tension.

 

However, when you convert drums to discs, you need to remove this valve, or your new disc brakes stay on when you let go of the pedal.

 

How does this work when you go the other way - putting a disc brake master cylinder on a drum brake car?

 

I see Wilwood sell aftermarket residual pressure valves, is this the only way, or can you install a valve in the new master cylinder?

 

JB1762 (15/16") bolts to KE30 booster. It comes from NE Pajero which had factory discs on the back. I'm also looking at Nissan master cylinders from S13 which had discs.

 

JB1608 (7/8") also bolts to KE30, but comes from from NA Pajero has drums on the back, so no issue there I guess.

Edited December 23, 2014 by Super Jamie

[lexsmaz]

I would think if the master cylinders that you talk about never had a drum option for them its unlikely they will have a machined area inside the outlet ports to fit the residual pressure valve & so as you said just go the Wilwood option & if my memory serves me right the main function of the residual valve is to actually hold pressure against the back side of the wheel cylinder cup seals so the stay seated / sealed against the cylinder walls ...

Edited December 25, 2014 by lexsmaz

[snot35]

Wouldn't the actual piston set up be the same between disk and drum brakes? I.e. if the sealing was a problem for drum brakes, then it would be for disks. I would have thought that the valve is probably holding the shoes close to the drum as there is a return spring between drum shoes. If they returned fully after brake application then you'd have to pump them a fair way before they'd actually provide braking, which wouldn't be ideal. I've never tackled the disk to drum problem, but it seems an after market valve should offer the least mucking around.

Edited December 26, 2014 by snot35

[altezzaclub]

Don't forget the handbrake mechanism holds the brake shoes outwards in a drum brake. As the shoes wear the one-way ratchet screws the shoes outwards to make up for it.

 

So you don't need a residual pressure in the line to hold the shoes out, in KE70s or a car with auto-adjusting drum brakes at least.

[Super Jamie]

Thanks guys, I'll keep the aftermarket valve in mind.

 

I guess the "block" on the firewall is just to split the rear master cylinder into two parts, not a residual pressure valve itself.

[lexsmaz]

All cars with drum brakes have an adjuster arrangement to set up the shoes close to the drums & if not working correctly the pedal travel is a lot more on the first brake application ..

 

Also the brake shoe return spring strength is far greater than any small residual line pressure & if not for the adjuster the pistons would travel all the way back in & the pedal on the first application would be down near to the floor..

 

Disc's & drums have a totally different seal set up & cannot be compared against each other ..

 

Personally if some of you guys are not to sure how it all works, my personal thoughts are better off not commenting & confusing the guy ..

 

Super Jamie, I have always come across the residual valves inside the master cylinder ..

Edited December 26, 2014 by lexsmaz

[altezzaclub]

I guess the "block" on the firewall is just to split the rear master cylinder into two parts, not a residual pressure valve itself.

 

..and also to contain the rear brake pressure limiter to stop the back brakes locking up.

 

Don't forget the discs on the rear need more fluid than the rear drums, so the 4-wheel disc m'cyl may be different inside to provide more fluid. You might end up with odd balance from the rears working too fast with the disc m'cyl on them.

Edited December 26, 2014 by altezzaclub

[Super Jamie]

Hah ok, so I might end up with a frankenstein of bias adjusters and residual pressure valves to convert an all-disc MC back to a disc-and-drum MC.

 

Sounds like it's a better idea to stick with a master cylinder which was originally for drums like the JB1608.