The Stovebolt Site Forums The Shop Area Engine Shop no front brakes?

This is on my 69, with front disc brakes (power). I went to drive it today for the first time in several months and barely had a brake pedal. The rear brakes worked fine, in fact I could lock them up if i pushed the pedal down hard. No front brakes. I have no leaks. The master cylinder is full. The truck only has about 1300 miles on it since built. I don't think bleeding the front brakes would do any good, because there are no leaks, correct? Is my master cylinder bad you think?

Hy oldcarandtrucknut, the combination valve may have shuttled over to shut off fluid flow to the front brakes when you stepped on the brake pedal for the first time in a long time, if you open a caliper bleed screw, fluid should flow even without pumping the brake pedal, loosen the master cylinder cover first. The calipers may also be siezed.

Well, I only have an adjustable proportioning valve on the back line. My dad said something about the calipers too, but if they were seized, wouldn't I still have a stiff pedal? Thanks.

Hy oldcarandtrucknut, in order for you to have no front brakes at all, the caliper pistons would have to be siezed in their bores, that may or may not present a symptom in the pedal. I assume you are using a single system master, if you only have a proportioning valve in your system.

no, it's a double reservoir MC, and the adjustable prop. valve is on the back brakes only. It's all custom, and it has always had a good firm pedal and stopped very well, until today. This is the first time I've driven it this year. I also used synthetic brake fluid when I built the truck (completed about 8 years ago), and I thought it was supposed to keep moisture out.

Hy oldcarandtrucknut, I think your master cylinder may be bypassing internaly. Synthetic brake fluid was designed for two purposes, raising the boiling point of the fluid, and it is not hydroscopic, it doesn't draw moisure out of the air as the alcohol based fluid will, consequently it makes it very apppropriate for vehicles that are not driven daily. If you have the equipment you could put a pressure gauge in the front system and see if you can generate any pressure in it, hope that helps

If you ran DOT 5 silicone fluid, you may have wiped out the cups in the master cylinder. Non-compatible rubber will dissolve/fail when used with DOT 5.

It does sound like the front brake rubber seal in the master cylinder could be letting the fluid by to the piston and seal for the rear brakes.

There has been a lot of problems here in the UK with silicone brake fluid being incompatible with the rubber components in classic car brake systems So I use a DOT 4 fluid and change it every year in my rides.

Stuart.

Did you use the DOT 5 fluid in a system that had formerly used another class of brake fluid? That would definitely cause problems with rubber parts (even if the rubber is compatible with DOT 5 fluid).

Thanks guys. The fluid I used is Valvoline synthetic Dot 3&4, NOT dot 5. It's compatible with conventional fluids, however when I built the truck all of the lines and components were new, so mixing it with old is not an issue. I'm going to try to bleed the fronts and see what happens. If there is little or no pressure, I'll replace my MC.