Hello. I have a '49 Chevy panel truck with a disc/drum system, and GM-type master cylinder and combination valve. Since my master cylinder is on the firewall, I know I don't need a residual valve in the front line, but do I still need a 10 lb in the rear drum brake line? I was under the impression that the master cylinder/combo valve had an integral residual valve - that's why it's for disc/drum, no?
Is there anything to..... 1940 ford rear brake = a pair of shoes, no leading edge bendix self energising physical aspect, no self adjusting cable 1954 f100 rear brake = a pair of shoes - self energising leading edge bendix - no self adjusting cables 1966 bronco rear brake = a pair of shoes, leading edge - self energising and self adjusting notes when looking at a schematic of a cut away valve says metering on the front and proportioning on the back firstly is the metering on the front adjustable sewcondly is the proportioning adjustable to the rear thirdly, where does it say residual? chronologically 1940 - is there a residual vlave in the 1940 single brake master? 1956 - is there a residual valve in the f100 brake master? 1966 - IF the bronco uses a single master - is it fitted with a residual valve in the master 68-72 f100 - dual brake master cylinder - is this dual brake master fitted with residual valves at BOTH ports? disc brake front debuts (roughly) with the 70-77 gm rotor = 11'' rotor, big single piston caliper, 1.1/8 master 11 by 2.5'' rear drum ? - rear drum self energising self adjusting cables holding the shoes within striking distance of drum.. QUESTION do these cars have a residual valves in the rear lines - they are not in the brake master, nor the combination valve/\ where is it on the exploded diagram? 78 up to 98 smaller metric single sliding piston caliper 10.5'' rotor, 1'' brake master, s10 with this has 9.5'' rear drums with a 2'' shoe - question do any gm cars from 78 up use a residual valve in the rear line? lastly - IF you are running 70-77 big size car front calipers and 10 by 2.5'' rear drums with a 1 1/8 brake master is the combination valve specific to this, regards metering and proportioning? so to answer the question, no idea and to add, there was a thread on here where the guy measured the pressure to the rear line 400 ilb's now regards rear brake lock up, IF for arguments sake the system front and rear is at 1000 ilbs , is the 400 real line pressure the 'point' on where the rear valve wont axctually open and flow fluid to the rear - or when 400 ilbs is seen at the front, the 400 ilbs acting on the spring opens flow to the rear brakes - so they don't come on first. proportioning - a pre set pressure to ensure the rear brakes wont lock up before the front. now - what is metering?
Most drum brakes up to the mid '70s used residuals until wheel cylinder cup expanders made them obsolete. I am not aware of any combo valves that include a residual, but typical disc/drum masters did have one located in the rear brake port behind the tube seat. Bottom line, if your drums are vintage, you need a residual, but adding a residual to later drums is perfectly OK.
As I stated above, I have never seen residuals in any combo valves. Single system (pre-'67) master cylinders had the residual located internally at the end of the bore, and were usually called a check valve. Cup expanders made the residual valve obsolete for most OEs in the mid '70s. Factory replacement combination valves should only be used on vehicles that came with them from the factory, NOT on any custom non-stock brake system. ALL you need for custom disc/drums is an adjustable proportioning valve and internal or inline 10# rear residual, regardless of master location. The 2# aftermarket disc residual is ONLY used when the master cylinder is lower than the calipers.
The metering valve delays front brake operation to give the rear brakes time to overcome the spring pressure so we don’t have a nose dive situation, generally blocks pressure to the front from roughly 30 psi to about a 100 psi depends on vehicle design. As Bob said unless the car you are building is the same weight as well as has the same front rear weight bias as the vehicle the combination valve came out of, it’s a bad idea to use it. Sent from my iPhone using H.A.M.B.
Thanks for your help gentlemen. My master cylinder is an aftermarket unit, advertised as "GM-type" for disc/drum, which I assumed had the internal residual valve.
Don't "assume" the residual will be installed. Always check to be sure, by gently inserting a small drill bit or similar into the rear axle master cylinder port. If resistance is felt, there is a residual, otherwise the inserted object will easily go into the bore. Some rebuilt or replacement master cylinders are very universal as they may come with a loose residual and tube seat to be used or not. Corvettes never used disc/drum, so the popular Corvette style master cylinder probably does not have a residual.
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