1957 NEW Power Boost,,,,,,,YES STILL NO BRAKES

[Walter Youshock]

It is a hydro vac system identical to 1955 and earlier setups.  The master is on the frame.  Pushing the pedal forces fluid from the master to the booster and the booster amplifies the pressure and sends that to the cylinders.

It's easy to reroute the master outlet to the brake line distributor block on the frame, thereby eliminating the booster from the system...

[TJ Hopland]

Getting the two systems separated then seems like the way to go so you know which part of the system has the issue.   

Is the MC part a fairly typical design piston with rings or is it like the treadle one with the check valve and the piston is displacing fluid?

The single drum systems don't move a lot of fluid and move even less air with each stroke so they can be more difficult to get working than disc ones that move a larger volume.

Is the brake light switch hydraulic on this?   That is a good place to trap air, you can try unscrewing it to bleed there.

The last one I fought with what I finally ended up doing was started shortening the system till I got pressure then started adding things back in.   They don't seem to make caps for brake fittings so what I did was bought some spare line and pinched off the ends.   I basically plugged the distribution block and could not get pressure there so I was able to focus on the MC.  Once I got pressure there I started adding wheels.  I got pressure in the front but when I added the back in I lost it again.  Turns out I had a bad new hose in the back in addition to a problem with the MC.    That was several weeks of frustration.    The MC may not have been bad to start with, I may have damaged it fighting with that rear hose issue.

[fishnjim]

Try Classic disc brakes.   The shop that did my work got the booster/master and mounting bracket there.
Should be the same as '58.   Need a proportioning valve to keep the drum brakes.   Several guys have done the conversion themselves.   It's a common GM part, but don't have the part # handy.