Drum/Power brake master cylinder - are they interchangable?

[Flatback72]

This post is related to my earlier post about if a 71-73 brake booster will bolt straight onto the firewall of a non-boosted drum brake car (the answer from Don of OMS was that it wouldn't without a different pedal and firewall plate).

So I now have a follow-up question: will a master cylinder (minus the booster) off a disc brake car fit on a drum brake without modification??

Once again to explain why I'm asking this - I'm thinking of putting a disc brake set-up on my car from an Australian Falcon which I know without doubt will bolt straight on. Rather than tracking down and installing a new pedal, firewall plate and factory brake booster I was thinking of getting my brake guy to install an in-line booster. However, a knowledgeable bloke on the Aussie Mustang forum pointed out that the drum master cylinder contains "a residual pressure valve in the drum circuits to hold line pressure", and that a disc brake master cylinder is needed because it doesn't have the pressure valve.

So if I remove the drum master cylinder, will a disc master cylinder without the booster connect up the same?

 

[Tnfastbk]

I would say 99% Yes. I am running 4 wheel maual disc brakes with a Wilwood MC and no proportioning valve. This set up now works wonderful since we lengthened the rod at the pedal some.

 

[c9zx]

IIRC the residual pressure valve for drum brakes is in the MC outlet port behind the flares seat and provides 10 lbs pressure to the drum wheel cylinders to keep the shoes close to the drums. If you are careful the flare seat can be removed with a straight splined broken screw extractor and the valve can be removed.

The piston in the power MC has a larger diameter (larger=less PSI for a given travel distance) and the hole that the pushrod goes into is shallower on the power MC so the pushrod length would have to be altered. The pushrod length is critical and tight tolerance (0-.010 movement of the MC piston when bolted down) Too short=weak brakes, too long= brakes engaged without pressing the pedal.

Using the drum MC (no booster) with the residual valve removed will require very high pedal effort and likely won't move enough volume of fluid to operate the calipers at max effectiveness. Using the disc MC (no booster) pedal effort will again be very high and likely won't generate enough pressure to operate the calipers effectively. Whatever you decide to do, proceed carefully on this one. Test and re-test before you take it on the road. Chuck

 

[Don C]

This post is related to my earlier post about if a 71-73 brake booster will bolt straight onto the firewall of a non-boosted drum brake car (the answer from Don of OMS was that it wouldn't without a different pedal and firewall plate).

So I now have a follow-up question: will a master cylinder (minus the booster) off a disc brake car fit on a drum brake without modification??

Once again to explain why I'm asking this - I'm thinking of putting a disc brake set-up on my car from an Australian Falcon which I know without doubt will bolt straight on. Rather than tracking down and installing a new pedal, firewall plate and factory brake booster I was thinking of getting my brake guy to install an in-line booster. However, a knowledgeable bloke on the Aussie Mustang forum pointed out that the drum master cylinder contains "a residual pressure valve in the drum circuits to hold line pressure", and that a disc brake master cylinder is needed because it doesn't have the pressure valve.

So if I remove the drum master cylinder, will a disc master cylinder without the booster connect up the same?

MP (Master Power) Brakes has a fairly good FAQ section that should help. http://www.mpbrakes.com/technical-support/reference-master-cylinders.cfm

I believe the aftermarket power disc brake conversion kits use the existing mounting holes and brake pedal, a little research may help to figure out how they do it, especially if you look at their instructions. The instructions for SSBC's are on Summit's website: http://www.summitracing.com/parts/SSB-A133-10/?rtype=1

 

[Flatback72]

Thanks for the replies guys - there's some good info here to discuss with the brake dude when I take my car back to him next.

I'm steering away from aftermarket kits because installing disc setups that aren't Ford standard issue requires an Engineers Report here in Australia, and I'm not really interested in having an engineer poking around my car and charging me $$$ for his approval. Of course I could install aftermarket brakes without an engineers cert but I'd be setting myself up for potential insurance grief if my car was involved in an accident and the car was inspected by the insurance company.

I'd get away using Aussie Falcon discs without needing an engineer involved because the parts are Ford and pretty much identical to the US OEM counterparts.

I might even settle for keeping my current drums and adding the inline booster just to make the pedal a bit easier...

 

[mustang7173]

Hello Flatback72,

Checkout Master Power Brakes.

http://www.mpbrakes.com/index.cfm

mustang7173

 

[gpierce]

So, just out of curiosity, why are you wanting to use the falcon parts? In my experience the actual mustang parts are cheaper even if you buy them in the US and have them shipped which I do regularly

Greg

 

[turtle5353]

I am running the stock manual drum brake MC on my car with disc brakes at all 4 corners and it works fine. I have the brake booster pedal and power mc off of a parts car that i am going to install this winter, just ran out of time last winter. But i have run my disc brakes with the manual drum mc for probably 5,000 miles so far and havent had any problems.