Brake proportioning valve - how does it work?

[relaxedcruiser]

Thank you for you thoughts.
My theory is that if there is air in the plug chamber and hollow part of the blue piston - the rear brake pressure will push the piston towards it. as it should per describtion of the function - but still I dont like the fact of air in the system.
However, I have managed to put it back together immersed and will try it tommorrow once bled properly - my first test drive just now showed still air in the system - so I have to do it again.
I believe that the brake fluid can't run out once the plug is closed as the cup seal in the plug itself is closing off that chamber.
By the way - all components of my valve were in really good condition, I have just replaced the two o-rings.

 

[Eurasiaoverland]

There is a tiny hole in the blue proportioning valve shaft, from memory close to the flange where the cup seal sits, presumably to release air from the hollow portion of the piston.

 

[relaxedcruiser]

So this is interesting - the brake performance on my truck was sort of intermittent quite good then went average then went quite good and so on. This really puzzled me for a good time - years actually. Then I did the above - cleaned the proportioning valve and put it back together with that little chamber filled with brake fluid - I sort of already knew what was happening now. The brakes are now always quite good, actually really enjoyable. But the proportion effect is gone - the flooded chamber prevents the piston from moving. The rear brakes are locking the wheels always before the front ones are even close to it. Not good. The piston must have been a bit stuck in the muck which was in there during times of "quite good braking.
The brake pressure point feeling is very positive now - which I highly appreciate... In my theory it can only be the now missing air in that chamber.
But this can't stay like it is, so I'm going to open that valve again and will but it back together with air in that chamber.
I'm curious how that will brake then.
And by the way - there is no little hole in that piston which connects that chamber with the rear circuit, at least not in my piston.

 

[BurntToast]

So this is interesting - the brake performance on my truck was sort of intermittent quite good then went average then went quite good and so on. This really puzzled me for a good time - years actually. Then I did the above - cleaned the proportioning valve and put it back together with that little chamber filled with brake fluid - I sort of already knew what was happening now. The brakes are now always quite good, actually really enjoyable. But the proportion effect is gone - the flooded chamber prevents the piston from moving. The rear brakes are locking the wheels always before the front ones are even close to it. Not good. The piston must have been a bit stuck in the muck which was in there during times of "quite good braking.
The brake pressure point feeling is very positive now - which I highly appreciate... In my theory it can only be the now missing air in that chamber.
But this can't stay like it is, so I'm going to open that valve again and will but it back together with air in that chamber.
I'm curious how that will brake then.
And by the way - there is no little hole in that piston which connects that chamber with the rear circuit, at least not in my piston.

Or replace it will a manual prop valve to your liking

 

[relaxedcruiser]

That is a option too, although I'll try first how it goes / brakes with the cleaned valve and air in that piston. Not too sure what the warant of fitness guy would think of those aftermarket brake additions. Here in NZ all vehicles built before 2000 needs to be tested for roadworthiness every 6 months (!)

 

[relaxedcruiser]

The brake pressure point feeling is very positive now - which I highly appreciate... In my theory it can only be the now missing air in that chamber.
But this can't stay like it is, so I'm going to open that valve again and will but it back together with air in that chamber.

So then I took that BPV out again, opened the plug and emptied the little hollow section at the end of the piston as well as the cavity inside the plug off the brake fluid. Put it back together .....and that is it ..... the brakes are working as they should again with the front locking up just before the rear.
(low tide at the local beach proved to be an exellent test surface) The brake pressure point is a bit softer now - It's a bit disappointing that the system needs air in it to work.... The otherwise very good describtion of the method of operation inside the BPV shows this chamber (arrow pointing towards it) in the same colour than those flooded with brake fluid, which is obviously not correct. So that's why the blue piston is hollow at that end - it needs the volume of air in it to be able to move...and the cupped seal is only there to keep that air inside... that it does not disappear over the decades ...is a bit of a mystery to me.
.

I wonder what will happen if I remove the little cup seal inside the plug....I have the feeling that the piston would be able to move - even with those chambers floded. But I might be wrong - as those engineers back then surely tested that. I might try that nevertheless at some stage.
That front mounted BPV seems to be amongst the very few things in the whole vehicle which are not covered in the WSM or (exploded) in the parts catalogue. The cleaning of mine was definitively neccesary - the brakes pressure point is constant now. And that was the main reason taking it apart in the first place.