Re: WANTED: NOT-rebuildable BTV master cyl.

Posted by HH56 On 2009/5/18 20:49:37

I'll go out on a limb and say there are 2 modes of failure possible in an otherwise undamaged unit--(hydraulic portion only). Hopefully PV8 will be able to confirm my theory.

One is the compensator valve. It is a fairly large piece of rubber molded onto a stem. The actual seal and contact area is approx 1/16 in wide around the top circumference--maybe 3/8 of an inch diameter. As soon as the piston moves, the valve spring has to pull and keep the valve squarely upright and hold it until fluid pressure builds and reinforces. If spring has broken or weakened and unable to pull upright quickly, fluid is pushed past valve and either all goes to reservoir or maybe a little stays but not enough for wheels. If there is any type of debris such as old crystallized fluid bits or dirt etc that is hard and large enough to land on the circumference and keep the rubber from sealing completely also same result. Symptom there is the dreaded pedal to the floor as you glide thru the stop light.

The other mode of failure is the seal between the piston and vacuum portion. While there are some scraper rings and assorted alignment washers, there is only a single lip seal. If the fluid has been neglected to the point piston has corroded or otherwise been damaged at the exact area where the seal sits, fluid will be sucked into booster--same with the seal if it has hardened. Symptoms here would be the obvious no fluid (assuming lines and wheel cylinders intact) and resulting brake loss. If refilled periodically while losing fluid, there could be a condition of hydraulic lock where enough fluid had accumulated in booster and wouldn't allow pedal to depress far enough. I think it would take a lot of fluid loss before it could be sucked into engine due to position of components.