Rear brakes locking up

[Lincoln7]

I am having an issue with my rear drum brakes locking up while panic stopping. My current setup is:
-Blazer front brakes w/ Powerstop Z23 pads
-Aluminum drum rears w/ large bore wheel cylinders for S10
-B Body large bore master cylinder
-New proportioning valve for disc/drums
-SS braided brake hoses
-Double diaphragm brake booster
-Vacuum canister

Suspension is in great working order: Bilsteins, Eibachs, Tubular control arms thoughout. I am running 255/40R17 front and 275/40R17 rear tires, same model.

I pull about 15" vacuum at idle and usually maintain 20-25" available vacuum while driving (vacuum gauge installed on A piller). I have bench bled the master multiple times, bled the system, adjusted the drums. I tested the pressure at my calipers and both read about 1500 psi. My braking has never been very good and now (since installing decent front tires) the rears are locking up during panic stops.

I have an adjustable proportioning valve I intend to install downstream of the stock prop valve to reduce pressure to the rear brakes. I am also considering installing C5 front brakes. Any insight on what could be causing this? I have been battling my poor brake performance since day one (about 15 years ago). I also notice my front brakes are dragging. I relieved pressure using the bleeder screw (to identify any bad hoses) but that did not help (on either side). Thanks for any help!

 

[malibudave]

Its more than likely the larger 7/8" bore manual brake S10 rear wheel cylinders versus a 3/4" bore wheel cylinder on g-bodies and power boosted S10s. Going from 3/4" bore to 7/8" bore is a huge increase in piston area for drum brakes. Adding an adjustable valve should help if you want to keep the 7/8" bore wheel cylinders, but it may be better to go back to the 3/4" bore wheel cylinders first and then add a adjustable prop valve if you need one then. Rear drums are a little harder to modulate over a rear disc systems, because rear drums are a little "touchy". For drag applications, drums are hard to beat for cost, weight savings, and holding the car on the line.

You could also run rear brake shoes that require more heat than a normal street shoe, but finding a "performance shoe" is almost impossible. If they make a ceramic rear shoe, that should be the best choice as they usually require more heat to work well.

You also may think of running a stickier rear tire. This will make it harder for the drums, or any rear brake system, to lock up.

With the B-body master cylinder, you should be able go with the Wilwood 2.75" bolt in calipers for the front brakes. This will give you more clamping forces, but you will still have the small diameter 10.5" rotor. Going with a Corvette front setup will give you larger rotors, but may not solve the rear lockup issue.

 

[Lincoln7]

Thanks Dave. I also have most of an LS1 F Body rear disc setup to install but I was thinking this would improve rear braking even more, making my issue worse. Thoughts on that compared to the drums?

 

[malibudave]

In my experience, going with LS1 rear brakes will not make it worse. Rear disc are just not as touchy as rear drums. Bottom line is that it is harder to dial in rear drums. Rear drums are almost "on" or "off" if the pressure and volume are higher as the rear shoes are being pushed by the wheel cylinders against the drum and pulled by springs away from the drums. Going up in wheel cylinder size, master cylinder size, and booster size will most likely make it more touchy or more easily "on" or "off".

Rear calipers work on hydraulics only, so they are easier to modulate with your brake pedal. The piston size in the rear caliper has a smaller piston area, so the force will not be the same as the front calipers.

What's easier?
Adjusting the parts to your rear drums. Smaller wheel cylinders, adjustable valve to restrict the volume of fluid to the rear drums, adding different shoes that have less friction cold.
OR
Adding rear disc brakes. Adding a disc/disc prop valve (bolt in with a universal prop valve). Hooking up the emergency brake mechanism. Depending on what the car is used for, adding a adjustable prop valve.

This all depends on what your car is going to do? Street, drag strip, steet/strip, auto cross, etc.

 

[Lincoln7]

Today I replaced the rear wheel cylinders for the factory G body 3/4" bore versions. I was only able to bleed it half arse but a test drive in the rain displayed very good results. The fronts were locking up only under very heavy braking, even enough to notice the rears locking up after the fronts only sometimes. Brake pedal feel was much better than I have ever felt in this car. The local NAPA had the cylinders for $9 each so this was a quick, cost effective attempt to get the car safe to drive until I'm ready to swap in the rear LS1 discs. Thanks so much for your help, Dave. I really appreciate your detail explanation and options. I was seriously doubting this Blazer brake swap I did like 2 years ago!

I will report back any change in performance once I have someone with a foot to help me bleed them properly.