85 W123 300D Testing Brake MC
 

Is there a way to check the Master Cylinder for the brakes while it is still on the car? My brake pedal travels way too much before the brakes start to grab. Panic braking doesn't make the car pull excessively to either side. Wheels will lockup.
I am suspect of my calipers as I can't find any leaks at the master cylinder. Calipers are old and rusty, pads still have good meat on them. I am planning to replace all 4 calipers with the Nugeon rebuilts from here. Should I just do the MC anyway? 199K miles but sits for 2~3 months at a time. Fluid is clean as I did a full flush when I changed my rear pads last year. I was hoping pedal would improve as the rears settled in, but no improvement after 3~4K miles.
Thanks

a sign of mc failure is slowly releasing pressure when you are holding the brakes, or of course, zero pressure when pressing on the pedal.

long pedal travel before braking pressure, can be a MC sign, but more often, it's a low fluid level, with air in the system.

I'll try one more bleed. I have the Motive thing but maybe I'll just ask my wife to do a pump-n-dump with me this time around. Pedal holds steady once the brakes are applied.

You could have the common problem with this design of master cylinder. The rear fluid holding compartment of the master cylinder has no fluid in it to speak of.

One nice feature of calipers that in general holds true. Wear is automatically adjusted for by additional fluid slowly added to the caliper cylinders as they extend further and further. So calipers cannot be the issue unless installed on the wrong side forever holding air in the offender as a result. The caliper bleeders must all be upwards in nature. Or at the high side of the internal fluid pocket in them.

I vote air or low fluid in that rear chamber although one part of the master cylinder may have failed.

A quick test is to jack up one rear wheel first. Have someone apply the brakes. Can you still turn the tire? Then test the front. Although in your case you stated you can still can lock four wheels. So on that basis the master cylinder is functional. The wheel test is not required. Of course verify all the wheels are really locking up on gravel or whatever.

I guess technically you could have a ballooning rubber brake hose but as an amateur I have never experienced this or heard of it.

Yes, you will have the system open.... just replace the MC with the new calipers....
not that expensive....and you sleep better at night...