The Stovebolt Site Forums The Shop Area Driveline ’49 disc brake conversion rupturing rear wheel cylinder

I completed the Front disc brake conversion on my 49 3600. Brake kit was from TSM. It came with a combination valve which has been properly installed. After a couple of short test drives in the neighborhood I noticed a puddle of brake fluid beneath the left rear wheel. Removed the drum and could see that the leak was coming directly from the wheel cylinder. Took the wheel cylinder off and inspected it, was older And a slight bit of roughness, not really pitting, on the inside. Ordered and installed two new rear wheel cylinders and when bleeding the brakes, had the same problem, the new wheel cylinder on the left rear began leaking. Took it apart and inspected it, cannot visually see any issues whatsoever with the wheel cylinder.
Questions:
1.) My initial thought is that there is excessive pressure going to the rear brakes causing the leak. When I installed the disc brake kit I was told that the combination valve has a built in “proportioning valve“ that reduces pressure to the rear drum brakes. Is this so? I’ve also read that it only functions in a hard panic stop and otherwise allows the same pressure to the rear brakes that flows to the front disc brakes, which I understand require more pressure than drum brakes. But, it seems odd to me that the pressure to the rear brakes would be so high that it would cause fluid to bypass the rubber cup and the piston in a way that caused a rather significant fluid leak, especially since the leak occurred during the bleeding process. Any thoughts or input on this?

2.) my current plan is to install a true proportioning valve in the rear brake line in an effort to reduce pressure to the rear brakes. In my research I can’t determine if proportioning valves vary in the amount of pressure that they will permit. For example if brake pressure in the amount of X psi is excessive for the rear drum brakes, it seems I would want a proportioning valve with an upper limit less than X. Does this make sense? I’m the first to admit that my knowledge in this area is extremely slim.

3.) Does anyone know of any other possible causes of the leak? It seems a little hard to believe that my new wheel cylinder is defective especially since I can perceive no issues with it upon visual inspection. I have read that with a disc brake conversion a supplemental proportioning valve is a must but have also read other threads saying that it is unnecessary and that the combination valve is sufficient to reduce pressure to the rear brakes. I’m kind of lost here, any help would be greatly appreciated!!

4.) As Long as I have your attention I attempted to bleed the brakes using my new Capri vacuum bleeder but all my efforts were somewhat disastrous. It mostly just sucks air and I tried numerous pressure settings between the minimum of 40 psi and over 100 psi. I wrapped the bleeder fitting with so much Teflon tape I could hardly start it by hand and had to wrench it most of the way in. Also tried fewer reps of Teflon tape but found the Capri bleeder to be wholly ineffective. Eventually went to the classic pump and bleed technique and that is when the leak appeared. Any similar experiences or any recommendations? Thanks, you guys have always been a such great help with my questions don’t know what I would do without this forum.

Most repair parts are made offshore, with little or no quality control, so it's likely you've encountered a bad run of new parts. I'd try rebuilding the new wheel cylinders with good quality USA-made repair kits, after making sure the pistons don't have excessive clearance. There's no such thing as "reducing pressure" to the rear brakes, no matter what kind of proportioning valve is used. The rate of pressure rise can be delayed, but the only way to actually change the pressure would be to have a master cylinder with two different diameter pistons. Most disc/drum systems have a spring loaded valve that delays pressure to the front brakes to allow for a little bit of rear brake to be applied in slippery conditions such as ice, snow, or mud without locking up the front wheels and losing steering control. Some pickup trucks use a proportioning valve that monitors rear spring squat to prevent rear wheel lock when running with a light load.

Most hotrodders don't have the engineering expertise to redesign their brakes, so they're at the mercy of the "kit" builders who are more interested in having a healthy bank account than actually providing their customers with a safe, reliable conversion. I have a tendency to be a little suspicious of anyone who promises to solve all my problems by selling me something! They might be selling "snake oil"!
Jerry

Thanks Jerry, that sounds like a good idea. Is there any vendor of brake rebuilding kits that you especially like or could recommend? Also, do the rebuilding kit come with new pistons? Thanks 👍🏻

Most rebuild kits don't include pistons. I've made slightly oversized pistons on a lathe at times, but that cut into the profit margin pretty severely! The rebuild kits have all the rubber parts such as cups, dust boots, etc, and usually come with a new spring and sheet metal cup expanders. Most wheel cylinder leaks are due to a slightly rough surface of the cylinder bore, or poor quality rubber in the cups. NAPA wheel cylinder kits are usually very good quality. Back when there were a lot of drum brake systems on the road, I had a stock of bulk wheel cylinder cups of various sizes, and if the rest of the small parts were good, I'd just hone the cylinders and replace the cups, instead of using complete kits. The cost of doing it that way was considerably less, and every penny I saved put more food on my kids' plates and kept a roof over their heads!
Jerry

Jerry –- what is a sheet metal cup expander? On the “new” wheel cylinders I purchased and on the one that failed originally, the spring rests directly against the rubber cup.

Also, any thoughts on my issue with the capri brake bleeder? There must be a better way than the old pump and bleed technique. Which, I admit, does work᠁᠁᠁

Some better quality kits use a cone shaped sheet metal piece between the spring and the cup, crimped to the end of the spring to spread out the load a little. I like "speed bleeders" where the bleeder screw has a spring loaded check ball incorporated into it, and sealing material on the threads of the bleeder. The screw is loosened slightly, and when the brake pedal is pushed fluid and air is forced past the check ball. On the return pedal stroke the check ball prevents air from being sucked back into the cylinder.

Most people pump the brake pedal too hard and fast during bleeding. One or two slow strokes of the pedal is plenty to pressurize the fluid, followed by holding the pedal down while the bleeder screw is opened. Pumping the pedal rapidly causes air to be "homogenized" into tiny bubbles that don't purge well. If the fluid at the bleeder looks foamy, your helper is getting too energetic on the brake pedal! I've been known to use a piece of broom stick propped between the seat and the brake pedal to hold it down when I'm doing a one man bleeding job.
Jerry

Did you have the brakes shoes adjusted out to the drums before you started bleeding. If they were too loose, the cups may have extended past the ends of the wheel cylinder. Or as Jerry pointed out, you may have received a bad batch of parts.

I adjusted the shoes so that I could barely get the drum on but I did not perform a classic brake shoe adjustment. I will definitely do that next time – I ordered a rebuild kit I’m going to take another shot at it. Will let y’all know how it goes.

Jerry, good point about the speed bleeders. As I was going through this process I thought to myself “someone should make A bleeder that has a simple ball valve to eliminate air leaking past the threads!” Lol as usual, they’re way ahead of me

Update, installed another new wheel cylinder, reset the proportioning valve, bled brakes with my new speed bleeders and᠁᠁.everything works great! Finally can enjoy the MUCH improved stopping power of my disc brake conversion. Bad “new” wheel cylinder was the problem, just like Jerry predicted.

Added note: rear brake shoes were soaked with brake fluid from leaking wheel cylinder and NO luck finding replacement for ‘49 3/4 ton rear shoes. Soooo, saw on stovebolt that boiling them for half hour or so in water and dish soap will clean them up. Some comments said don’t bother it doesn’t work, get the shoes re-lined. Well, I’m here to tell you it does work – and very well at that! I boiled them outdoors in a turkey size pot for 30+ minutes, dumped the black water and repeated the cycle. Let them dry, gave a blast of brake cleaner and a rough up with sand paper and they are good as ever – good stopping and no grabbing. Thanks to those experienced readers for the old school (and inexpensive) fix. Love Stovebolt.