The Stovebolt Site Forums The Shop Area Engine Shop More on brakes...Moron brakes

The other brakes post was timely as the task before me this morning was to bench bleed and then install the master cylinder into my 63 chevy after a disc brake conversion.

I spent a couple of hours bench bleeding the master cylinder with some custom bent fittings/line from a donor truck and then bleeding each cylinder.

I still can't get the brakes to pump up or get any pressure to the back brakes though. I got frustrated and had to give up for the day.

At this point, since the master cylinder, vacuum booster and proportioning valve are from the same donor rig I got the front suspension from, I'm thinking I'm better off starting with new ones anyway...

...especially in light of the extremely poor color and large amount of debris in the old fluid. Also, in between removing my bench bleeding fittings and screwing in the brake lines I could hear air being sucked up past the check valves.

Is my pedal not pumping up and holding pressure indicative of a bad vacuum booster or am doing something wrong again?

If nothing else, I can say I learned alot this week.

Sounds like you have a bad or plugged master cylinder. There shouldn't be any vacuum holding in the master when bench bleeding. The piston in the master is getting held up by crud either interfering with the piston movement or blocking the fluid orifices. I would buy a new master and start fresh.
Scott

"NEW" master cylinder, they are cheap compared to your life or the life of others!
Any dirt and crud,,,REPLACE IT!(If I had the money, I would buy it for you)

Ditto on the new M/C!
Have you also rebuilt/replaced wheel cylinders, etc.? Older brake systems have a habit of springing leaks when they're subjected to the operating pressure of a new master cylinder, so it's best to do a full rehab while you're at it. Don't forget the rubber flex lines to the front wheels and the rear end, as they can collapse internally and hold pressure in the brakes, resulting in wear and/or pulling to one side when the brakes are applied.

I like to do a flush of the brake lines, etc., with alcohol to remove accumulated crud and old fluid, then do enough bleeding to purge any leftover alcohol out of the system. Since brake fluid is made of alcohol and glycerine, plus other additives, the alky won't damage the rubber parts.
Jerry

Ditto. ditto all above! How about the steel brake lines? Are we talking new? If not, maybe use air to clear the lines, blast the internal gunk into a pail disconnect hoses and clear out one at a time. Use inverted flare plugs so no crud is blasted other than the pail. Inspect lines and possible pressure test. If all checks out, then flush bleed as recommended by others.
Otherwise: make new good lines of known quality. The residual pressure checks are not intended to block two way flow only to retard somewhat the flow back to the mc.

One more comment- - - -
Are the front brakes drum, or disc? The check valves are ONLY used for drum brakes, to keep the wheel cylinder cups from sucking air by retaining a little positive pressure in the lines. The shoe return springs retract the shoes from contact with the drums. If a M/C with a check valve in the front brake port is used on a disc brake system, it causes the disc pads to drag and wear, because the caliper pistons don't have return springs. Be sure you get the correct M/C for your application! On a disc/drum system, the combination valve requires about 100 pounds of pressure buildup in the front brake lines to prevent front wheel lockup and loss of steering control in slippery conditions. Sometimes it's necessary to manually release the spring-loaded stem of the combo valve to get pressure to the front brakes during bleeding.
Jerry

good stuff, thanks.

Unfortunately for me the nearest parts store I trust is an hour away AND I had to order it for delivery on Monday.

I also ordered a new vacuum booster. Before I go and pay for the thing, is there a way I can bench test the used one? Or am I just better off buying the new one?

...also...

I was in the process of converting from drum to disc brakes when I ran into issues with the donor master cylinder. I am attempting to run disc in the front and drum in the rear, just as with the donor vehicle. I ordered the master cylinder and vacuum booster that correspond to the brakes from the donor vehicle.

I know I am showing my naivete about this stuff again but, how do I rebuild the wheel cylinders?

A wheel cylinder is just a cast iron piece with a precision-bored hole, with aluminum pistons and rubber seals to receive the pressurized fluid and transmit the force to the brake shoes. Start by removing the shoes, and pulling the rubber dust boots off. Don't worry about saving them, the rebuild kit will have new ones. Some dust boots are a press fit into the cylinder, others overlap the outside diameter. There should be NO fluid inside the dust boot, it's only there to protect the piston. Push the pistons and seals out, clean up the cylinder bore with alcohol and a clean rag, and inspect the bore for rust pits. Sometimes, the cylinder will be too badly pitted to recondition. There is a brake cylinder hone available to polish the inside of the cylinder, but if you don't have one, use a piece of wet-or-dry sandpaper and some brake fluid for a lubricant, and polish the inside of the cylinder. Use a rotating motion, so you don't create lengthwise scratches in the cylinder bore. Wash thoroughly with alcohol to remove all the polishing grit. Clean up any rust or corrosion on the pistons with the same fluid-lubricated sandpaper, and wash thoroughly. Reassemble with the new seals (cups) and the old pistons, with the sealing lips of the cups pointing inward. Lubricate the cups with fresh brake fluid during the reassembly. Most wheel cylinders have a spring with a cup expander on both ends to help the cup keep a tight seal against the cylinder wall. Snap the dust boots on, install the pushrods that mate with the brake shoes, and reinstall the shoes. Be sure to get all the springs and other parts back in the right place. Some guys like to leave one side assembled while working on the other, for a reference, but remember each brake will be a mirror image of the other side!

Simpler, and quicker method: Buy new wheel cylinders, and bolt 'em on! Bleed the brakes, and you're done!
Jerry

When it comes to the most important safety item on a ride, new is always the best option.
Scott

hehe..i guess there is such a thing as 'too frugal'.

Jerry, that was very good. I'm a brake pro specializing in the restoration of vintage brake parts, and the only thing I would change is: *some* wheel cylinders have a spring with a cup expander on both ends instead of *most*...it depends on the engineer who designed the rubber cups, and in my (rather considerable) experience the expander type cups are in the minority.

Scott, new cylinders are often but not always the best option. Depending on the cost of the new parts and the length of time one expects to own the vehicle, sleeving wheel and master cylinders can be more economical, and definitely offers longer service life, than new cylinders, especially the cheap off-shore repops that are so common in the vintage parts catalogs. Good quality Stovebolt wheel cylinders are still available at reasonable prices, but most new Stovebolt masters cost considerably more than the price of sleeving and a rebuild kit and won't last nearly as long.

Joe

Thanks, Joe!
Since I teach in a city school system, I seldom, if ever, rebuild brake parts. There's always some ambulance-chasing lawyer itching to hit the school system with a big lawsuit, and one mistake on the part of one of my students could have multi-million-dollar consequences. Back when I had a real job, I used the Tru-Torque repair parts, and they had the expanders in all their kits. Never had a failure on a cylinder that was rebuildable, in my opinion, but I was pretty picky about cylinder condition.

For anyone who's considering rebuilding brake parts, remember you've got your life, and the lives of other motorists out there, in your hands. Nothing is as scary, and potentially disastrous, as a brake failure out there on the road! 100% perfect is just barely good enough!
Jerry