47-59 brakes locking up

[nvrdone]

So I got to spend some time on the truck today. After a short test drive I remembered why I didnt really like the brakes. On a hard stop they lock up.
Some time ago I replaced the stock m/c with a corvette didc / disc one with 10# pressure valves in both front & rear lines. Yes, Im still using stock drums front and rear. Tried backing off the rear brakes hoping that would help. No luck.
Then I thought about an adjustable proportioning valve in the rear line to limit the pressure.
Has any one done this and what were the results?
Thanks

[dsraven]

which brakes are locking up, just the rears?
maybe a little run through on what you have for system parts would be good to know. do you have a prop valve now? if so, what from?
do you plan to install discs all around at some point or what was the thoughts on using a disc disc master cyl on an all drums truck?
are you using the stock frame mount master cylinder mounting spot or a firewall mounting location?
are your drums brakes adjusted properly all around? is there a park brake and is it also adjusted properly? can you run us through the drum brake adjustment procedure you use?
have you checked the drum brake backing plates for the dreaded worn out spot where the shoes rub against the backing plate? sometimes that can be an issue for getting the shoes to adjust properly because the new shoes with more friction material drop into the slot worn in the backing plates from the old worn out shoes.
have you checked to ensure the ports on the master cylinder are connected properly as far as which is front and which is rear brakes?

[geezer#99]

I used a wilwood adjustable prop valve for my 56 210. It worked well to limit the wheel lockup. Just took a little testing.

[leegreen]

I'd confirm there is nothing wrong before using prop valve to limit your braking. Sounds like rear is locking up first.

Do the front brakes even work? that would be the first thing to check

Your avatar says this 49 has a M21, so open driveline, so not factory rear end. You probably have Bendix self energizing brakes from a newer chevy - not Huck brakes from a 49. Are the shoes on the right way? The shoe with the smaller friction surface needs to be the shoe on the front at each wheel, if the larger shoe is on the front it will make the brakes grabby.

Are the brakes adjusted properly, with good firm pedal well off the floor.
If they are not adjusted properly and the shoe is making first contact near the leading edge they will be grabby.

Are the shoes arced to fit the drum? Most stuff is sold pre-arced to fit a new spec drum these days. For new shoes properly adjusted you want to see contact on the center-trailing edge portion. As above if the contact favors the leading edge they will be grabby. For worn in shoes you want to see even wear over the whole shoe, if the wheel cylinder edge of shoe is worn more they have not been self adjusting, if it is bad then you need new shoes. If by some supply chain fluke you got un-arced shoes the linings are probably oversized and hitting only at leading and trailing edges....makes em very grabby.

Are the drums round? Do they have rusty patches on the friction surface from sitting? Does the pedal pulse at all?

How hot are the drums just driving? Drums should not be hot unless you have been using them, if the brakes are dragging and getting hot, a bit of heat can make them grabby. Too much heat makes them fade / useless. If your residual pressure valves are overcoming the shoe return springs the brakes will drag. If the brakes are over adjusted they will drag. If the ebrake mechanism/cables is sticking they will drag

So you have a disc-disc master - it will probably have both reservoirs the same size.
Disc-drum has a larger reservoir for the front that the back....and using it with 4 wheel drums may very well result in the front brakes coming on sooner than the rear, inside the master the vent ports between reservoir and bore are positioned so the front starts to build pressure before the rear as disks need more fluid volume, but the lower fluid volume needed for the drums may mean the disk end of the master is building pressure before the port for the rear is sealed. So if you have disk - drum master and front/rear reversed over how the master is designed you might have the rear brakes coming on sooner than front.

Some disk-disk masters might have brake bias built in with a stepped bore size, you wont want to use one unless you need brake bias.

Are you running a booster with a '49 original bore size master? it will make brakes more sensitive on the pedal, but should not create a rear bias.

FWIW I am not a fan of residual valves for drum brakes except in the case where a lowered car with under floor master has the master lower than a wheel cylinder or disk caliper in which case I would run a low pressure residual like 2 psi.
back in the day many cars with front disk had residual valves in the master for the disc end to keep the pads from being pushed too far from disk by the disk fluttering but I don't think that is common any more, master cylinder flow volume bias is built in to counter it.

[qbeanie]

I had a similar problem after replacing the master cylinder. A bench bleed was performed and the unit installed. Rear brakes locked as you described. I rebled the mc on the vehicle using a bleed kit Ashe
that resolved the locking.
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[mr48chev]

My experience says that the rear will always lock up first on a Chevy or GMC truck.

I've got the same adjustable proportionging valve that goes in the rear brake line for my 48 that Geezer #99 mentioned in post 3. Someone had it on sale one day and I snagged one. I'm not sure if I bought it for the 48 or the 71 as I locked up the brakes on the 71 when a Geo Metro ran out in front of me one night and spun the truck around in the road but missed him. If I had been gawking off he would be dead. The truck spun so violenty that it broke the alternator bracket.

back in 1989 or 90 I rollled my 70 when a gal pulled out in front of me and I spun out missinng her. The back locked up and spun around on it too. Drum/drum on that truck.

[dsraven]

is the master mounted on the frame or the firewall?
got a pic of the master? is there also a power brake unit?
lots of good points made above. make sure all the peripheral stuff is working like it should and adjusted like it should be. also, a few answers as to what you have for front and rear brakes may help. if the rear axle was replaced, what have you got for an axle and brakes? drum diameter, wheel cylinder size and show width? what are you using out front? what master cylinder are you using, specifically?

[dsraven]

sorry, not show width, shoe width. you prolly figured that out though.

[dsraven]

maybe check the pushrod that actuates the master cyl. there should be some play in the pedal before the master cylinder piston is contacted and starts to move. if you have a booster sometimes the master and booster are not set up properly so the master cylinder doesn't get to return all the way, there is a few youtubes on adjusting the booster to master pushrod. if it is also frame mounted there is some linkage and pivot points that can wear, stick or bind and that can cause issues. if you can see down into the master cylinder, like the cap is big enough to show the reservoir area, then you could look in there, from a distance of course-no brake fluid in the face that way, and see if there are 2 little fountains or disturbances in the reservoir fluid when a friend steps on the brakes. this happens because when the piston in the master is allowed to return all the way back it opens the vent hole in the bore and allows fluid to return to the reservoir as needed or allows fluid to be replenished in the bore as needed. anyway, long story short, when the pedal is first stepped on the piston moves ahead and displaces fluid but the fluid would rather return to the res than go down the line and it can do that for the first little bit of stroke of the piston, until the piston passes the vent hole. this fluid movement causes a little fountain in the reservoir fluid, or at least a disturbance in the fluid, so that tells you that the piston has returned far enough back. if the piston is not returning fully then it can be like holding your foot on the brakes pedal and this gets worse as the brake fluid heats up and expands, which takes more volume in the system, which can compound the problem.
there are different res valves for different types of brake set ups but, basically, if you have a firewall mounted booster the res valve pressure would be a little different than a frame mounted master. its about the weight of the column of fluid in the brake line at the different heights above the wheel cylinders. the wheel cylinders need a little retained pressure in the line to keep the seal lips pressed against the wheel cylinder bores so they don't leak fluid out or contaminants in. a lot of master cyl's made for drum brakes had the valves built into the master cyl outlets, disv brakes ones didn't. some rebuilt masters for disc brakes are actually old drum brake masters and some have the res valves still in place. tells you the kind of rebuild done, lol.
anyway, a little more info on your actual system would help. pics would tell a thousand words.

[chevyguyase]

Originally Posted by geezer#99

I used a wilwood adjustable prop valve for my 56 210. It worked well to limit the wheel lockup. Just took a little testing.

I did a front power disc brake conversion on my '54 3100. The first time I drove it in the rain it went sideways. I used the wilwood adjustable prop valve to cure it. I kept the one that came with the kit and added the wilwood valve. Some instructions say not to use both, but it worked for me. It stops fine now. Test drive it on a wet street so it won't surprise you like it did me.

[dsraven]

good idea on the test with wet streets, especially if you live where there isn't much rain because when it does rain all the stuff dripped on the road becomes like grease when it gets wet.
aside from doing some tests with expensive tools you don't have much for options besides ensuring it is connected properly at the master, ensuring it all works as it should and is adjusted the same at all 4 wheels (asuming 4 wheel drum), all the tires are inflated close to the same, nothing is sticking (park brake cables), auto adjusters work, etc. after that it is a matter of trying an adjustable prop valve to see if you can get things to work together.