Board logo

Fiat Uno bias valve - Adjust which way for more rear brakes?
craig1410 - 31/3/09 at 12:16 PM

Hi,
I know there are loads of threads talking about the Uno valve already but I haven't been able to find one which states categorically which way around the valve works. ie. whether restricting the plunger's movement increases or decreases brake pressure to the rear.

My SVA is on Friday and I am keen to set my bias to a "reasonable" guesstimate so that I can hopefully just drill it and fit a rollpin after the brake test. Also, if it doesn't pass then I'd like to know which way to adjust it and how sensitive the valve is to maximise my chances of reaching an acceptable bias level before the SVA inspector runs out of patience.

If anyone know for sure how this valve works then please let me know. I have seen threads by knowledgeable members which state that it works in both directions and I have my own opinion too but what I need is practical advice based on experience.

Many thanks in advance,
Craig.

britishtrident - 31/3/09 at 12:39 PM

The valve only works one way -- it can only reduce the presuure.

This type of valve works in a way quite similar to gas bottle regulators the plunger needs pushed and held in to in to increase the pressure to the rear line.

Edited -----Corrected senior moment

[Edited on 31/3/09 by britishtrident]

matt_claydon - 31/3/09 at 12:40 PM

I have one but can't remember now. Easy enough to work it out if you know how it was fitted to the Uno - it is designed to reduce pressure to the rear as back of the car lifts and the suspension extends.

craig1410 - 31/3/09 at 01:01 PM

quote:
Originally posted by britishtrident
The valve only works one way -- it can only reduce the presuure.

This type of valve works in a way quite similar to gas bottle regulators the plunger needs pushed and held in to in to reduce the pressure to the rear line.


See I thought they worked the opposite way by the plunger going in providing more preload on the spring which prevents another internal piston from closing the fluid ports to the rear brakes. That way screwing in the plunger would increase pressure by keeping the ports open until higher pressure is achieved.

How sure are you?

Thanks both for your responses.
Craig.

t16turbotone - 31/3/09 at 01:13 PM

hmmm.......from memory i think that when you do not limit the travel of plunger you get max pressure to rears, when you limit the plunger travel, lower pressure, mine has a gap of 1/2 inch or so!

James - 31/3/09 at 01:13 PM

I have an uno bias valve. I didn't fit a restrictor to the plunger on mine for SVA. Just left it open.

Passed fine with new Sierra front and rear brakes.

HTH,
James

[Edited on 31/3/09 by James]

theconrodkid - 31/3/09 at 02:18 PM

it,s not a a matter of pushing the plunger in,its a matter of stopping it coming out too far,the further it comes out the lower he pressure to rear brakes....complicated

craig1410 - 31/3/09 at 02:41 PM

quote:
Originally posted by theconrodkid
it,s not a a matter of pushing the plunger in,its a matter of stopping it coming out too far,the further it comes out the lower he pressure to rear brakes....complicated


Hi conrodkid,
This is how I would expect it to work. Are you sure about this? Any idea what sort of gap is likely to be needed with cortina front discs and sierra rear discs with a sierra m/c. More importantly, any idea of the sensitivity or range of pressure reduction given?

Thanks,
Craig.

MikeCapon - 31/3/09 at 03:22 PM

I do not know how this particular valve works but I may be able to help you.

I presume you can't set up the bias by driving the car but you can get a pretty good set up without taking the car out of the garage. We used to do it this way to get a start point with adjustable bias on race cars and sidecars.

You need a friend for this. With the car up on stands, get your chum to slowly increase pressure on the pedal until the rear wheel(s) are only just possible to turn by hand. Then check out the fronts. Adjust and repeat until you are happy.

I know this sounds odd but it does work.

Cheers,

Mike

stevegough - 1/4/09 at 08:03 AM

I can't help at all - but just be glad you are getting it SVA'd as the IVA (comes in at end of the month, I believe) requires you to WELD the locknuts AND the rest of the thread - making the bias permanently fixed.

What happens if you've got the set up wrong? Not something that is easily undone, is it???

And its fee is set to be £540 to boot!

Anyway, I wish you good luck with the test.

Best regards, Steve.