[Rovernet] 3500s brake saga

[magnet at ican.net]

Hi Peter,

I've gone through the P6B brake experience too...

One thing I remember was that one of the "new" TRW master cylinders I 
fitted came without a gasket between the cylinder casting and the air 
valve housing.  This let air slowly into the "controlled " side of the 
servo vacuum chamber and thus prevented the brakes from staying released 
fully.

I also had a problem with a vacuum hose that got soft when it got hot, 
and partially collapsed under vacuum, keeping the vacuum from getting to 
the controlled (front) side of the servo diaphragm, preventing the 
brakes from releasing promptly when one took one's foot off the pedal.  
I'm sure you used proper vacuum hose -- fuel hose won't cut it.  I was 
unable to find vacuum hose of the correct size, and had to use copper 
tubing.  This was a PITA to bend, but certainly has made the brakes more 
responsive.

I don't know if the kink you mention in the reservoir hose would be 
enough to prevent over-pressure in the system from sending fluid back 
into the reservoir, but, again, hoses get softer when hot and the kink 
might get worse when hot...

I'm sure you've checked the amount of free movement at the master 
cylinder push rod with the brakes released... To some extent this is a 
function of the  thickness of the gasket between the m/cyl and the 
casting it mounts on, but I rather doubt this is the problem, as I can't 
see how it would be temperature dependent.  I'd still look for ways that 
air could leak into the vacuum control circuit.

Good luck, keep us posted on how you get on!

Cheers,

Bill  Daddis



On 03-Aug-20 12:46 PM, Peter King via Rovernet 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.
>
> 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?
>
> Thanks for any thoughts.
>
> Peter
>
>
>
>
>
>
>
> 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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