47-59 brake question

[nvrdone]

on my '49 3100 im in the process of completely rebuilding the brake system:
ie new master, lines & hoses, wheel cylinders, springs, shoes & drums.
Previously I've had a problem with the back brakes locking up on hard stops & causing a skid.
In doing some research ive come to understand that smaller bore wheel cylinders means more pressure at the shoes, larger bore means less pressure.
Right now ive got 1 1/8 " cylinders in the front & 1" in the rear. If my understanding is correct, that means that im getting more pressure to the rear brakes.
So my question is first, is my understanding correct and second, for those if you who are using front & rear drums, what bore wheel cylinders are you using and third, can you provide me a source and part number?
thanks for all the help.

[mick53]

I would think you could fix that with an adjustable proportioning valve but I'm no expert.

[mr48chev]

I've had my 48, my son's 70 that I totaled six months after he died, and my 71 have the rear brakes lock up and spin out. It's far more a too much weight in the front and too little in the back than it is wheel cylinder size.

It would be a lot simpler and easier to put an inline proportioning valve in the line to the rear brakes than tinker with wheel cylinder size. They aren't too expensive nor too hard to hook up. Something like this one from Jegs even though it is on Amazon.
https://www.amazon.com/JEGS-63020-Ad.../dp/B078WJDLCS

[mick53]

I have been told the front should lock up slightly before the rear. Also find some wet pavement to start with at about 25 mph. Have someone watch you. When you get your bias where you want it move to dry pavement at slow speed to tune it and then up your speed. The key is to have an observer. Maybe take a video for better examination.

[Wrenchbender Ret]

Its the opposite. Large bore wheel cyls. give more pressure. Small bore gives less pressure.
There are a lot of difference in brake shoe lining friction. Do you have the same type linings on front & rear?
George

[nvrdone]

yes have the same size & type shoes front & rear. I thought smaller bore = more pressure,less volume. larger bore = more volume, less pressure.

[dsraven]

according to rock auto the rear bore size should be 1.188"

https://www.rockauto.com/en/moreinfo...1486933&jsn=13

fronts would be 1.25"

https://www.rockauto.com/en/moreinfo...33&jsn=9&jsn=9

if redoing the system you might think about a master cylinder upgrade to a dual system as it is a safer system should anything go awry. with the original system the master cylinder supplies all the brakes using a single hydraulic circuit. if a leak happens then the system runs dry all of a sudden with only the park brake as a back up. on the dual system the fronts and rears are 2 separate hydraulic systems so a leak would only run 1 circuit dry and you still have the other circuit to get you stopped.

if using the original style single circuit master cylinder then the pressure in the system is the same, front and rear, unless there is a prop valve installed in the line to regulate the line pressure to the rear brakes.
not to confuse you but there is basically not much pressure in the system until all the brake shoes contact the brake drums and you apply more pedal effort or hold the same pedal effort on the brake pedal. the master cylinder just provides a flow up to that point to move the wheel cylinders out. that is the reason why the pedal moves down to a certain point fairly effortlessly, because there isn't much resistance to the flow of fluid in the system because the brake shoes have not contacted the drums yet. once the shoes contact the drums and there is really no more flow required then the master cylinder starts to apply brake pressure according to the pedal effort applied by the operator. once that happens then the difference in brake shoe pressure against the drums, between front and rear, is the difference caused by a system pressure acting on a smaller surface area on the rear wheel cylinder pistons compared to the same pressure applied to a larger piston out front.
make sense? it's like a hydraulic jack. a big jack ram diameter moves the jack less per stroke of the pump handle but is able to lift more due to the size of the jack piston. it is hydraulic pressure working on an area. a larger capacity jack is just a bigger cylinder diameter than a smaller capacity jack, most would not have a noticeably different "pump" size, just a noticeably different ram size. with smaller wheel cylinder bores out back the shoes will contact the drums first out back because the cylinder is smaller so the same amount of fluid displaced moves the piston through it's bore quicker, but won't generate much brake force until the front shoes contact and PRESSURE starts to build in the system. pressure doesn't really build until the wheel cylinders encounter a stop in movement with fluid still being displaced by the master cylinder.
due to the laws of hydraulics the fronts will generate more force, for the same pressure applied, than the rears will because the front wheel cylinder pistons are larger in comparison. the rears will contact the drums first because the pistons are smaller and take less fluid to move the same distance.
maybe this will explain it better. scroll down to the post by schurkey, second to last on the page. think force instead of pressure.

http://www.v8buick.com/index.php?thr...r-size.294254/

[dsraven]

for awhile the manufacturers were using valves on the rear brakes that used a link from the frame mounted valve to the rear axle, a lot like an air ride valve would have. this was to limit the amount of brakes applied relative to load on the rear axle. if the axle was closer to the frame, like a big load was being carried, then more brake pressure was available (if applied by the operator). if there was a lot of distance between the axle and the frame, like the truck was front end loaded from a panic stop or possibly going over a big bump /airborne, then the pressure was limited to the rear brakes. this helped stop rear wheel skid. some had pressure valves as well that limited the rear brakes if a lot of pressure was applied to the system. this would help stop the rear wheels from skidding under heavy brake applications like an emergency stop. a skidding tire grips less than a tire that is trying to slow down or has resistance. it's the theory behind antilock brakes.
anyway, a lot of upgrades happened to brake systems since these old trucks were first built. they were also used mostly for work and not really made as speedy, smooth riding vehicles.

[mr48chev]

Stock rear axle or changed to ????

[nvrdone]

front axle is 3" dropped I beam. rear is 12 bolt from a '75 c10. napa shows stock wheel cylinder for c10 is 1" bore which I have & is what original axle info shows. front is 1 1/8" which is what I have.
So maybe the best solution is to install an adjustable proportioning valve in the rear line & work from there.
I was just wondering what wheel cylinders others with front & rear drum brakes were using. the sizes just seem backward.

[mr48chev]

Do you have the 2 inch rear shoes or the 2-3/4 inch shoes on the rear. 2-3/4 coming on trucks rated over 6000 lbs meaning camper specials or tow packages. Real common in the mid 70's. Those trucks could also be had with 11x2 shoes but I don't think they can be swapped without swapping backing plates and drums.

Original rears on that truck were 11x 1-3/4.

[MiraclePieCo]

Tire size is another factor. Larger rear tires mean less weight per square inch on the pavement, so less resistance to skidding, which exacerbates the already inherently nose-heavy weight bias of a pickup.

[nvrdone]

hey raven - thanks for the info. I have converted to a dual master cylinder with a power booster. the kit is from performance on line.
mr48 - the rear axle is out of a 75 c10 and both front & rear shoes are 11x2.
miracle - tires could be another issue. fronts are 215/70/15, rears are 30x9.50x15.
maybe I should just load about 5 sand bags in the rear for balance.
Im at the point of looking into a proportioning valve.

[nvrdone]

so ive done more work on the brake system. After bleeding the brakes, I left the truck on jack stands with the wheels off. next day I found a fluid leak at both rear wheel cylinders. seems the w/c I got from napa were metric thread, not sae thread and were seeping. so back to napa. after 3 tries, got a good set. they are in & will be bled today. hopefully no leaks.