To boost or not to boost
 

Recently, I rebuilt the rear calipers of my 71 280se. I attempted to bleed the calipers without using the booster (engine running) but did not have much luck. All kinds of scenarios began to pop up: master cylinder bad, blocked line, etc.. I decided to crank and when I did I was able to bleed the brakes with no problems at all...:confused:

You will also want to change the flexible brake lines if they are more than 10-15 years old. They swell and close up over the years and won't allow the brake cylinders to release they way they should. I incurred the unnecessary expense of installing rebuilt calipers before discovering that they weren't the problem--it was the hoses.

The hoses are a good place to start.
I have seen some vehicles which have a metal clamp to secure part of the hose.This can rust internally and effectively pinch the hose.
A few particles of crap can clog up the proportioning valve,if so equipped.

Thanks..
I am in the process of re-doing the system-new hoses and the like but to be honest I had not thought of the restriction part of it. My main concern was having to deal with no braking as a consequence of a hose bursting and leaking fluid. I am having to use plenty of 'parts blaster and WD-40' in order to loosen the hose fittings. Also, a PO had some brake work done on the front calipers and the bolts holding the calipers to the suspension must have had 200lbs. of torque on them. According to the manual I have all one needs is 82 ft.lbs.:)