Brakes are in. however, I get a lockup, or at least drag on rear wheels. Trying to determine if the master cylinder has a built in residual valve. corvette style same size reservoirs, supposed to be for disc/drum as I have. (power assist).
I don't need to install an inline residual valve (10 lb) to rear wheels if so. under floor applic. as original. This could be one explanation. I would like to finish brakes and get to tranny shifting properly, who knows, maybe start driving it around someday. little victories keep me going.



1951 f1

 

I thought that the same size reservior indicated that the MC is either a drum/drum or a disc/disc. The disc/drum MC's have different size reserviors since the front discs require more fluid to actuate vs. the rear drum systems.
I think that some of the MC's had built in residual valves but some also used an external combination valve that included the residual valves, the proportioning valve and on the drum/disc, the valve that delays the application of the front discs to avoid severe nose dive (can't remember what that valve is called). Anyway, if you do have a disc/disc MC with built in residual valves, it would not be the correct valve for the rear.

Good Luck!

 

If you have a MC under the floor you need a residual valves to prevent fluid bleeding back to the MC. If MC on the fire-wall you don't need residual valves. I don't think they are in MC. Proportioning valve has a metering valve within itself. Google brakes for a complete description of system.Others will clearify soon.Have a great day, chuck

 

Thanks Chuck. I know I need a residual valve...what I don't need is two going to the rear brakes. Hence, determining whether m/c has one built in. I have a GM combo/distr. valve which does the proportioning (front to back ratio) and metering. Under floor mounting has inherent pain to work on, or to ascertain its interior components. Will get under there and check for built in residual valve with a small probe. Rolling under the truck stopped being fun twenty five years ago. found nice drawing at mpbrakes.com pretty much what I am trying to do. Basically aftermarket hotrod brake system with power disc/drum application. just been a pain in the butt to work out the details. One Day at a Time

 

P.S. part of what messes with my mind is that the after market m/c's have equal size reservoirs, most people of my age grew up with m/c that had large & small reservoirs. large = front, small =rear. I'm getting an education as I work on the system. If m/c is correct, and it doesn't contain a residual valve, as I am told they tend not to have, I will keep the 10 lb in line residual valve. Then I am inclined to check the rear shoes for fit to drums. Re bleed,and check next possibility. it is usually the minor problem that I overlook that turns out to be the culprit. Slow and Steady wins the Race...

 

While you are under your favorite place see if you can find a MC part number or some way to id the MC so you can check with the manifacturer for info. Seems like all articles at FTE everyone adds the valves. I think you need one in the front and rear, they act as check valves. Mine is a firewall mount, MC mounted higher than the brakes. Once installed and bled you should be ok. Have a great day,chuck

 

about your rear wheel lockup - Is the proportioning valve adjustable? It may be that the gm combo valve is setup for a car which has more weight over the rear wheels. You may need an adjustable one so that you can dial in the front/rear bias.

 

1. A Master Cylinder for Drum Brakes will have a residual valve built in. If you bench bled the MC, you would have noticed its function.
2. Someone said another wrong answer: " The Disc reservoir is larger because it requires more fluid to acturate" The answer is: The disc system does not return, the pads stay out on the disc, and as they wear the piston stays extended further out, and this is what requires more fluid. If you never add flluid (you shouldn't) to the reservoir on the disc side, you can tell how much wear there is. When you put in new pads and return the piston, the fluid will return to the reservoir. (If you had added fluid to fill the disc reservoir when the pads were worn, the disc reservoir will overflow when the disc piston is returned).
3. The reason for the residual valve is to hold the drum brake cylinder pistons at a low pressure so that they do not completely return. Otherwise, even with shoe adjustments (manual or automatic) it would take extra strokes of the brake pedal to move the shoes to the drum. While a firewall mount would not allow a gravity return of brake fluid to the drum reservoir, no residual valve would still allow the drum brake cylinder pistons to completely return. The residual valve also stops any gravity return of fluid to the reservoir when the MC is under the floor and possibly below the wheel cylinder pistons. (usually only on lowered trucks).
4. The proportioning/metering/alert valve is only on disc/drum setups and is there for two reasons: To reduce pressure to the drum brakes which are self energizing (Bendix) and on an unloaded truck might easily lock up; and to actuate a switch if either the disc or drum lines have no pressure. On some trucks there is a single valve, on others it is done with two separate valves. When building a truck, I always use an aftermarket adjustable proportioning valve. This allows me to reduce the rear brakes to a level that is correct for the truck at its normal loading. It also allows me to set more rear braking when it is loaded and towing. When your truck is driving in snow or on dirt, it is important to have correct rear braking to avoid unwanted lockup and subsequent spins.
5. A 4WD truck, with all its rotating mass is very difficult to lock up any brake, front or rear, but it is nice to have the braking balanced, as brake lockup will be almost impossible while in 4WD. This is also why 4WD trucks do not need 4 wheel antilock systems, providing they are in 4WD.

 

Hey Thanks Guys. good points. After I went ahead with the GM combo valve, I learned some more...wish I had gone with adjustable proportioning valve. And I do need to get some facts, starting with the m/c. Payday was today...think I will go ahead and switch to adj. pro. valve on rear wheels. have 2lb. residual in front. I would like to tune braking based on truck weight, load, conditions,etc. I MAY have been excited & didn't bench bleed the m/c enough/correctly. Time to exhale, and do some work in a methodical order. will keep you posted, I'm sure more questions will arise. T.G.I.F.

 

I agree, I think you have the wrong MC for your application.

 

A disk/disk MC will work for disk/drum and drum/drum applications. Just don't use a drum/drum for disk/drum.

 

Not true because a disc/disc has piston sizes that are going to be too small which will not move enough fluid and will cause higher pressures than a drum can use. Pistons for discs are under 1". Pistons for drums are around 1 1/2"
There is a good Bendix M/C for Disc/Drum that uses a 7/8" disc piston and a 1 3/8" drum piston. It is still made and is the best for GM and Ford (Bendix) Discs and any Bendix self energizing drums.

An adjunct to this is the fact that if one sizes the disc piston correctly, no power assist is necessary. Discs need about 1200 psi to lock up and about 900 to stop quickly. Drums only need about 300psi to lock up, and also need more fluid. A typical Bendix sliding caliper disc has a 3" piston and the proper MC piston size is about 3/4" . This is a 4:1 advantage. The pedal mechanical advantage is about 10:1, so 85 lbs on the pedal yields 1200 psi.
Power assist is used because the piston size is not always optimum, and they want the pedal pressure to be less than 40 lbs. (can we say overboost?) A normal non assisted pedal yields great stopping power and very good modulation, much better than a low pedal with boost. I use aftermarket master cylinders which are singles, then they are bolted together with a linkage bar which accepts the pedal actuating lever. These M/Cs have a large amount of piston sizes so you can come up with what you need. They also help you choose the sizes if you tell them what size your caliper piston is and what size your rear wheel cylinders are.

Regards,

Alanco

 

Alanco, Thanks for the info. I am learning/making adjustments as I go. it's a kick in the butt to have to do things twice, or buy parts twice, but in the end-I want the brakes to work well, and be durable. I have to rethink how I am going to finish brake system. Truck is going inside for the cold weather...chance to roll under it and do it right. live and learn.

Chilione

 

The hardest part is letting go of driving it this year. Was hoping to. now I have to refocus and get it ready to drive out next April 1st. Just a delay. maybe optimistic planning. : )

 

more food for thought...I am under the impression that i should be running 1/4 " line to the rear drums. right now, 1/4 line ends at hose connection. 3/16 hose and to wheel cylinders. Can I get a 1/4 line that will fit my wheel cylinders? stock 51. my solution was to terminate 1/4" line at hose with adaptor. this is another variable. Shoestring budget doesn't help. Research shows that a m/c # mc390387P would be apropos my setup. Going to work on truck...Patriots don't play until 8 P.M. : )