[Rovernet] 3500s brake saga

[Vern Inkspot]

Comments interlined

> On Aug 3, 2020, at 9:46 AM, Peter King via Rovernet <rovernet at rovernet.org> wrote:
> 
> I Rovernetters. I’ve posted this on the Federal Rovers group on Facebook, so if you are on there this is repetitive.
> 
> I have been trouble shooting problems with my 71 3500s brakes. I’ve installed a new master cylinder, rebuilt the servo, replaced the flexible lines, tested all the vacuum lines for leaks, replaced the check valve at the servo and both seals, rebuilt the servo 3 times. Flushed and bled the lines multiple times, following the procedure of both rear and front left side together, then right side, then close up the rear. All bleeding done into jars of fluid. 
> 
> When I start the car cold, I have equal vacuum pressure on both sides of the servo. Brakes work well, hold and release when the pedal is depressed.  BUT, what seems to be happening is that as the engine heats up, pressure builds in the system and the master cylinder air valve gets forced open. No spring would be strong enough to compress it. As a result, the vacuum from the master gets weaker, as air is let into the servo. And the brakes seize. 

Right from the beginning the 3500S seemed to be plagued with this issue. A new air valve was always part of the solution but I also recall that a master cylinder rebuild/replacement was part of the puzzle.
> 
> So the questions is, what could be causing this? The master is new, so unless it arrived faulty I am not suspecting it. yet. I've replaced the flexible lines. I get fluid bleeding fine through all 3 bleeders. I do have a kink in the master fluid reservoir inlet line... could it be preventing the master to recover quickly, and therefore cause a lock? could it be that as the fluid heats up,  it has no place to expand (back to the reservoir--I don't know if it does this normally)  and is causing the lock? What else am I not thinking of?

The master cylinder is open to the reservoir at rest. But that depends on the cylinder piston(s) being at the far stop. I’d make sure the pedal pushrod is adjusted so that is so, if you’re holding the piston(s) down a little bit what you suggest might be possible.
Was the master cylinder NOS? If so, it might be worth a teardown & inspection to confirm everything is serviceable. I’m thinking congealed rubber grease for example blocking something.

Yours
Vern

> 
> Thanks for any thoughts.
> 
> Peter
> 
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> 
> Peter King + Company
> 10 Derby Square
> Suite N2
> Salem, MA 01970
> 617-292-7877 x302
> www.king-co.com
> www.facebook.com/pkingco
> 
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