[al37ford]

Didn't find this on a search, likely because its too simple I have inherited a front disc/rear drum power assy. in my '49 F-1 and never having had to redo this likely '80's Mustang assy before I have a question. Taking the cover off the underfloor master cylinder, there are 2 rubber cones, attached to the cover, that dip down into the fluid catchments. The front one seems normal shape while the rear one is compressed up tight to the cover. Is this cause for concern? I was wondering ifd it means the rear isn't working or else the prop. valve isn't right??

[61bone]

This diaphragm is to seal the fluid from the air to keep from contaminating your brake fluid with moisture. As the fluid level changes when you apply the brakes the diaphragm moves with the fluid level. As the front brakes (on front only discs) wear, the fluid level drops as the piston moves out of the bore. The large chamber of the M/C runs the front brakes in a disc system so it would be normal for it to be pulled down and the other not. If it is the other way around I would look for a leak in the diaphram. In reality, unless you keep the M/C full to the top both will be pulled down some.

[al37ford]

Good info, thanks!! I assume then that if I purge & flush the system & replace the old fluid with the new silicone fluid, the diaphrams would be redundant?

[61bone]

No, there is still dirt and water to consider, especially in an underfloor arrangement. while the moisture is not absorbed into the silicone fluid, it can still destroy your system by corroding components internally if it gets into the system. Dirt in the system will wear it out at an accelerated rate. In short, do everything possible to keep your system clean and dry. I have never had an underfloor but if I did I would consider some sort of boot to cover the M/C.