1950 Chevy 3100 pickup, drum/drum, original front, late model rear. Installed dual master cylinder, original location, good pedal, 1/2" (roughly) freeplay. * While bleeding, good fluid stream at rear, less at front wheels. * Stops on road test, will not lock up, not as good braking as with original m/c. * Brake shoes (new)& wheel cylinders checked out good, adjusted "by the book" ? Do I need residual valves for this setup? Suggestions?
tell us about the brake pedal...did it change when you changed the master cylinder? If the ratio is different, that would explain it.
I think if you shared which master cylinder you are using, may get more help. Possible you may need to change the length of your pushrod? There is a chance someone has already done this swap. Thinking of doing the same with my Dodge .... seems 91-94 Toyota Camry has the same 3 bolt pattern & will bolt in. I will need to shorten my push rod a hair ... this Master cylinder does have residual valves built in. I only know this because it has been done many time before.
Master cylinder purchased from Cl***ic Parts of America, got stuff from them since back when they were Chevy Duty. This m/c is for this application (47-53 Chevy truck conversion). No residual valve built in that I know of (not mentioned in description). New brake hoses front and rear, m/c came with adjustable push rod, followed instructions that came with it to set it up. Everything else is stock/not modified. Will install residual valves, didn't realize they would be needed with drum/drum. Thanks to all, you guys are fast !!!
Yes. If the master cylinder had them, the cones where the flared lines seal would be br***, and not whatever material the main body of the master cylinder is made of. Also, 1/2" of free play, I am going to ***ume that is at the pedal pad? If that is anywhere but there, that's a problem.
Don't know original m/c piston bore size. ***umed that, since this m/c advertised as replacement/upgrade, it would be ready for this application. (The 1/2 " Freeplay is at the pad)
Did you also install new brake shoes front and rear? I have heard of a proceedure called "re-arcing the shoes". This is done to match up the radius of the shoe surface to the radius of the (possibly turned???) brake drums. Without re-arcing it is possible the shoes are making only point contact. Since you've already driven it why not pull a drum and look at the contact patch witness marks?
Heard of that. Will pull a drum and check it out. (Note:. Everything worked great with original m/c, all new parts installed just before change-over. Seems problem appeared after New m/c install)
Just a thought from a old carpenter that is not a mechanic. My OT Chebby truck has disk/drum brakes. Because of a blown rear wheel cylinder .... I learned the large reservoir feeds the front brakes, the small reservoir feeds the rear .... need more fluid for the front disk brakes. I know you have drum/drums .... You say the front is not getting as much fluid as the rear. The rear reservoir on the master cylinder feeds the front on my chebby. I just wonder if it is possible you have the lines crossed, even drums the front brakes are 60% stopping power. I would think your master cylinder would have 2 pistons with 2 residual valves .... they may not be equal value. One for the front, one for the rear. <---- just a dumb carpenter. Something to check ..... if your lines are correct, your large reservoir is in the rear like my newer chebby. Possible you need to adjust your rod longer to get more throw .... if it is not fully activating the rear piston. Arcing the shoes to fit the drums is important. The goal is to get 100% surface contact from shoes to drum. You may only achieve 70% contact, but every square inch of contact is better braking. Just not sure how it affects pedal travel though .... If you have 50% now, you improve it to 70%. You will stop faster but pedal would not change if adjusted properly?
Never ***ume. Low braking power may well be due to a larger bore in the new, moves more fluid faster, but less PSI per lb of leg pressure.
If it is this master cylinder: https://www.cl***icparts.com/1947-59-Dual-Master-Cylinder/productinfo/71-201/ This might be the problem. That master cylinder has a bore of 1-1/8", and is generally intended to be used with a power brake booster. The original master cylinder has a bore of 1". That is a comparatively huge difference.
Note, if this is the case, this is a generic Corvette-style master cylinder, and they can be had with a 1" bore, so a replacement would bolt right in.
That's the one. To be clear, are you saying I need smaller, 1" bore m/c ? As well as residual valves ?
Even with bench bleeding, new lines, and all new master and brakes, I've still occasionally had an air bubble trapped at the outlet line fittings that needed to be bled. I just have someone press the pedal and then break the fitting loose at the master and then tighten it back down. That all it takes to purge the air bubble, and then the brakes worked great, and bled as they should.
A proportioning valve should solve your problem. Get an adjustable one. You want a little bit more pressure on the back brakes than the front. The back should contact the drums first. At least that's the way it used to was. A residual valve only keeps a little pressure in the system. That's all it does. Drum brakes use a ten pound residual, which only reduces the amount of pedal travel and time before the system reacts. It does nothing to help distribute the flow/pressure.
I've always found the opposite. You want less brake pressure at the rear drums than you have at the front. If the system has more pressure at the rear the rear tires will lock up first and the rear end will come around in a panic stop. Need more front brake than rear to stop straight. So both of my cars use adjustable proportioning valves to reduce pressure to the rear and stop straight.
The way I understand proportioning valves is when the pedal is applied pressure is applied to the rear drums first, before significant pressure builds in the front discs (drums). This keeps the car going in a straight line. The pressure only rises in the rears to a predetermined level, not full-on MC pressure. As that is happening pressure is built up in the fronts and that pressure exceeds the pressure in the rears because roughly 80% of the stopping power is done at the front brakes. The Adjustable Prop valve allows you to vary these ratios. With a non-adjutable one the ratio is fixed.
The roundy round guys used to call that bedding. This ^^^^ is good advice by the way. easier to diagnose a problem if you can find the problem.
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