Master Cylinder or something else?

[BaldEagle]

Good morning folks -

Before I change out the master cylinder, i wanted to run this by everyone else and see if i'm missing something -

in my 5 speed D1 - Under any normal driving condition - brakes are fine (as fine as a Disco can get at least). but any time i am in a lower gear at higher RPM's for an extended period of time (i.e. going up steep hills on mountain roads/trails) the brakes go to the floor initially. they are quickly pumped up (one pump), but nonetheless, not good. i've never had to make any vacuum system repairs so i'm not too familiar with that. i feel like the high rpm's are related, as that is the only other common denominator. any thoughts?

 

[listerdiesel]

Check the non-return valve in the line from the inlet plenum to the servo, plus the pipe itself and the connectors at each end.

On the D2 it is part of the pipe itself, and is formed at the servo end where it mates to the servo chamber.

I've got a complete pipe here on my desk and can confirm that the valve is definitely there, slightly different part on the D1 but serves the same function.

Other options exist, it could indeed be a faulty master cylinder, what sort of mileage has the car done?

Peter

 

[BaldEagle]

Thanks for the tip. What specifically am I checking on the return valve? Is there a test other than a swap out? The vehicle has north of 250k miles - its much like an airplane nowadays as many parts have been replaced time again - but that is original I'm certain.

 

[listerdiesel]

Just try sucking at the servo end of the pipe and see that it is working as a one-way valve.

Peter

 

[discostew]

But if he lost vac. I would think the pedal would be hard when he hits it. Maybe loose wheel bearing pushing pistons back into calipers and first brake apply is pushing them back out against the rotor.

 

[listerdiesel]

Lots of possibles, the wobbly disc is another good one to follow up, but I'd have thought that would have been fairly noticeable when driving?

Peter

 

[BaldEagle]

Lots of possibles, the wobbly disc is another good one to follow up, but I'd have thought that would have been fairly noticeable when driving?

Peter

after doing some research on the vacuum no return valve, it looks like i have the opposite of that symptom. but i keep thinking this is somehow vacuum related.

wheel bearings are fine. it only occurs after/while ascending for long periods of time, or driving in low gear without stopping for while - anything with prolonged higher engine rpm's.