Brake Bias Valve Probs

[jonah 1]

I have an adjustable brake bias valve fitted, which I bought from Compbrake. When I first fitted it I set the adjustment knob to the middle of its range (it has about 7 turns between min and max). Ever since then I've been gradually turning it towards "decrease" whenever I've noticed the rears locking. But I've now reached the end of the adjustment and the rears are still locking before the fronts under hard braking

 

Anyone else had this problem (or indeed any success) with the same... errr... "brand" of bias valve? I don't actually know who makes it as it's got no name on it... it's just a block of black anodised ali with a knob on the top, two holes through it for mounting, and a hydraulic port on each end marked IN and OUT. I've seen the same one advertised on ebay and various other places so it seems to be quite common?

 

btw I'm running standard brakes with 1155's on the front, and rear discs with standard pads. All the brake components themselves are in good condition so I'm pretty sure it's not a problem with ineffective front brakes or anything like that.

[smckeown 1]

sounds like the valve is nackered. what you describe is fronts are getting 100% and the rears are getting far less, and still locking up. I assume in the middle is where both get 100%, i.e. no bias; that's how I understand it anyway

[B1ack_Mi16 67]

sounds like the valve is nackered. what you describe is fronts are getting 100% and the rears are getting far less, and still locking up. I assume in the middle is where both get 100%, i.e. no bias; that's how I understand it anyway

 

But if it's split fron / rear I think that may be the problem.

 

I have done a front/rear split on mine with brembos on the front, and got the knob style Wilwood valve.

I don't think it'll reduce the pressure by more than maximum 50%, and that may be too little when running it front/rear split.

 

If you run it diagonally split I think it'll be better, but then again you can't adjust front/rear without mounting two valves and adjust them the same.

[Craigb 2]

Jonah

 

My set up is the same as yours , and the front to rear split is perfectly acceptable the way yours is plumbed in .

 

The valve only reduces pressuer to the rears , it does not affect the fronts .

 

My suggestion is wind the valve fully to the front and try braking , but be ready for it when it snaps away . then try and adjust it back , if you don't get any adjustment , then i think you have a faulty valve.

 

Worth a check on the fronts while you are at it . Just to be sure they are working 100%

 

Craig

[SPGTi 2]

I would check all joints both front and rear (esp front) to make sure you haven't got a leak anywhere esp when the system is pressurised. I ran without any compensators to the back for a long time and it never really felt overly braked to the rear.

I now have a very similar bias valve plumbed in and run with it about 3/4 open and it is fine. The only time I have had problems was when I retapped a rear caliper bleed nipple which leaked after a few good yanks on the bar meaning I could only lock 1 rear wheel.

 

steve

[jonah 1]

Thanks for the replies guys... Craig / Steve, do you have the same make of bias valve? And can definitely get it to lock the front wheels first on dry tarmac? This is the one I've got (doesn't have RL on the side though).

 

Definitely no leaks in the system, and I'm sure that the front brakes are fine because the stopping power is excellent apart from the rear lockup issue. The type of hydraulic circuit split definitely doesn't affect front / rear bias (this has been done to death in previous threads)!

 

I was wondering if this particular make of bias valve doesn't have a low enough pressure limit for the 205 with rear discs (considering how light the rear end is), but if other people have got it to work then mine must be faulty.

[Craigb 2]

Mine is not the same make . has no markings on it apart from something in italian , can't remember what it says though !

 

I have 180 brakes on the front of mine though , so they could be providing a better balance.

 

I seem to remember Steve thinking that the 180's were overkill on a 205 , See what Steve posts.

 

 

Might be worthwhile going to an MOT station and noting the braking forces that there are generating ?

[Butler 0]

Surely if you wind the valve fully in it should stop the rear brakes from working at all.

 

Sounds like the valve is not working.

[SPGTi 2]

Mine looks the same. I it got off US ebay. Mine seems to work as at the moment I have basically shut off the rear brakes as I have fluid seeping out of a bleed nipple. I am loosing a lot less fluid setup like this than when I had the valve about 3/4 way open.

I am getting the rear caliper sorted this week so will let you know what happens then.

 

Yeah Craig I found that with the 180 front brakes when on looser stuff I was locking the fronts up too easy.

 

Steve

[sam1312 0]

i have jus baught a wilwood 1 i will let you kno how i do.

[B1ack_Mi16 67]

Surely if you wind the valve fully in it should stop the rear brakes from working at all.

 

Sounds like the valve is not working.

 

This ain't they way they work, at least not the wilwood one.

The Wilwood one will reduce the pressure with maximum 57% according to their website.

 

Case is that if you have front brakes that have pretty big pistons, and rear ones have small pistons in comparision, when you then do a front/rear split of the system, the fronts will generate less pressure because they'll need more fluid movement to get the same pressure as the rears.

 

So in a front/rear split you'll probably need to reduce the rear ones pretty much, and maybe at least more than the maximum of 57% the wilwood one will do. I have a wilwood valve, so will see how it works out when I get my car sorted.

 

How about you get a fixed rate pressure-reducer and mount in series with the adjustable one you already have got? That should probably help on the problem.

[jonah 1]

Case is that if you have front brakes that have pretty big pistons, and rear ones have small pistons in comparision, when you then do a front/rear split of the system, the fronts will generate less pressure because they'll need more fluid movement to get the same pressure as the rears.

 

So in a front/rear split you'll probably need to reduce the rear ones pretty much, and maybe at least more than the maximum of 57% the wilwood one will do.

 

Sorry but I can absolutely assure you that that's wrong! Pressure is equal at all points in a closed hydraulic circuit, regardless of where the circuits are split - that's one of the fundamental principles of hydraulics. Volume of fluid movement is pretty much irrelevant when talking about brake bias. This has been discussed before (see the pinned thread at the top of this forum), but basically the type of split (front / rear or diagonal) has no effect on brake bias - and also larger caliper pistons generate more braking force, not less.

 

Interesting about the "maximum pressure reduction" though, I'd assumed these things would just be a hard limit at a certain pressure. So it means that it doesn't reduce rear brake pressure as much as I thought it would. Could just be that with my current combination of brake pads and tyres (it's got cheap no-namers on the rear), there's just a little more rear braking force than the tyres can cope with... will have to try swapping the tyres round.

 

Steve when you say 3/4 open and fully shut off, which way direction do you mean on the adjuster knob? And do you have standard front brakes on your car?

[B1ack_Mi16 67]

Sorry but I can absolutely assure you that that's wrong! Pressure is equal at all points in a closed hydraulic circuit, regardless of where the circuits are split - that's one of the fundamental principles of hydraulics.

 

But afaik the front and rear outputs of the mastercylinder is two different circuits.

You'll have the same amount of piston-movement in both of them. So if the front one will supply fluid to calipers with big pistons and the rear circuit supplies calipers with small pistons I can't see why the pressure shouldn't be higher in the rear one?

 

There is a little fluid movement (not much) when applying the brakes, and the fact that the pistons need to move a bit before they grip just means that the front circiut will need to supply more fluid than the rear one to get the same pressure buildup.

 

I talked to a local brake specialist and he said it was normal to run mastercylinders with for example 20mm for the rear circuit and 23mm for the front one, just because of this.

 

I perfectly understand that in a diagnoally split system (like ours are ment to be) the pressure will be the same in the front and rear calipers.. But I really can't be convinced that's the fact with front/rear split and the same diameter on both circuits in the MC...

 

EDIT:

Ok, I think I have misunderstood the way the MC actually works.

If it's like the front and rear circuit are connected under normal operating conditions the pressure will of course be equal in the whole system. This connection between the front and rear circiut will then close when pedal movement exceeds a given value, and therefor ensure pressurbuildup in at least one of the circiut if one of them fails?

Edited June 6, 2006 by B1ack_Mi16

[Craigb 2]

Had a look at my adjuster last night , and lo and behold, it is not a pressure reducer , but it is a flow control.

 

Now i don't now about the others on the market , whether they adjust flow or pressure , but mine is definalty a flow control .

 

Made by an Italian company , FT or FR can't tell from the logo.

[SPGTi 2]

Steve when you say 3/4 open and fully shut off, which way direction do you mean on the adjuster knob? And do you have standard front brakes on your car?

 

Will have to check the direction, but mine is labelled open and close. So at the moment mine is in the fully closed position. Before I had a leak it was set at (starting from fully closed) 12 turns open (where fully open is 16 turns). I am running std brakes all round with a hydraulic handbrake kit (uses std handbrake not a standalone).

 

this is the one I have Bias valve

 

Steve

Edited June 6, 2006 by SPGTi

[B1ack_Mi16 67]

Casing is a bit different to the Wilwood one, but also that one say 57% maximum reduction.

[jonah 1]

But afaik the front and rear outputs of the mastercylinder is two different circuits.

Yes

You'll have the same amount of piston-movement in both of them.

No, the two pistons move independently. There's a good explanation here. The pressure in the primary circuit generates a force against the secondary piston, which in turn generates pressure in the secondary circuit. The pressure in both circuits will be equal even if the volume of fluid movement is different in each (unless the volume is so much that the secondary piston meets the primary piston or the end of the cylinder bore, which should only happen if there's a leak).

 

I talked to a local brake specialist and he said it was normal to run mastercylinders with for example 20mm for the rear circuit and 23mm for the front one, just because of this.

Not sure what he meant by that... with a balance bar assembly the pedal force is proportioned between two master cylinders, but because the bar can swivel, again it can allow for different amounts of movement on each cylinder without affecting how much force is delivered. So different size MC's just give you a ratio of pressures between the two circuits.

 

EDIT:

Ok, I think I have misunderstood the way the MC actually works.

If it's like the front and rear circuit are connected under normal operating conditions the pressure will of course be equal in the whole system. This connection between the front and rear circiut will then close when pedal movement exceeds a given value, and therefor ensure pressurbuildup in at least one of the circiut if one of them fails?

No, the circuits are never connected - they're always separated by the secondary piston. But since the secondary piston is normally floating, the pressure will be the same on both sides of it. If a circuit fails, then the secondary piston moves all the way in one direction and meets up with either the end wall of the MC or the primary piston, so that there's effectively only one piston to pressurise the one remaining circuit.

 

Had a look at my adjuster last night , and lo and behold, it is not a pressure reducer , but it is a flow control.

Craig I have no idea how that works then!! Fluid flow is normally considered negligible in a brake system, and it only flows while the brake pedal is actually moving, so restricting the flow would have no effect on brake bias with a steady pedal force.

 

I've looked up other makes of bias valve (Wilwood, Tilton, AP) and they all control pressure not flow - some even have graphs on their website of input pressure vs output pressure. How do you know yours is not a pressure reducer?

[Craigb 2]

How do you know yours is not a pressure reducer?

 

i don't , all i know is that the hydraulic sysmbol on the side of the adjuster is one for a flow control not pressure reducer .

 

Theory , little flow , but i do know that if i screw it in all the way i have no rear brakes at all ...

[B1ack_Mi16 67]

No, the circuits are never connected - they're always separated by the secondary piston. But since the secondary piston is normally floating, the pressure will be the same on both sides of it. If a circuit fails, then the secondary piston moves all the way in one direction and meets up with either the end wall of the MC or the primary piston, so that there's effectively only one piston to pressurise the one remaining circuit.

 

Understand how it works now, searched it up on howstuffworks and they had a nice flash animation showing how the first and second pistons are connected with a spring that equals out the pressure

[Pugnut 9 1 Cars]

what i'm about to say could possibly be absolute rubbish (jonah has previously proven this)

 

but from what i've read of jonahs own preachings is that if you upgrade the front brakes, as i see it even with upgraded pads alone then you need to upgrade the rears .

 

you have started the valve off in the middle of its range and always worked downwards. Reading that a lot of guys also run with no pressure reduction on the rear with no problem should you maybe be adjusting the valve so that you get more pressure to the rears?

 

are you shifting too much weight forwards making the rear light.

 

there you go i've probably just made me look a right dork

 

 

Al

[steendp 0]

Could just be that with my current combination of brake pads and tyres (it's got cheap no-namers on the rear), there's just a little more rear braking force than the tyres can cope with... will have to try swapping the tyres round.

 

Last season I drove with less than premuim tyres on the back, with good tyres at the front and standard brakes. The back end was almost uncontrollable, so, as you are suggesting, try swapping the wheels.

[jonah 1]

what i'm about to say could possibly be absolute rubbish (jonah has previously proven this)

 

but from what i've read of jonahs own preachings is that if you upgrade the front brakes, as i see it even with upgraded pads alone then you need to upgrade the rears .

 

you have started the valve off in the middle of its range and always worked downwards. Reading that a lot of guys also run with no pressure reduction on the rear with no problem should you maybe be adjusting the valve so that you get more pressure to the rears?

 

are you shifting too much weight forwards making the rear light.

 

Yep you're right Al..... about the talking rubbish bit! (sorry )

 

Yes it's true that for the best stopping distance you need the correct front to rear bias, and upgrading the fronts alone will upset this, because the fronts will tend to lock up too easily while the rears will still have grip left that's going to waste. If the bias is wrong it'll be the end with too much bias that locks up first - in my case the rears. So I definitely need to reduce the pressure to the rears (or fit grippier rear tyres) to solve my problem. Which is why I'm wondering if my bias valve isn't working as reducing pressure is what it's supposed to do!

 

As I've said before, upgrading front brakes can't increase the maximum weight transfer when you're at the brink of locking the wheels, because the maximum braking effort is always limited by the tyre grip.

 

If people are running no pressure reduction on the rear without problems, then I can understand it if they also have uprated front brakes, since that will increase front bias and make the rears less likely to lock up. But if it's with standard brakes then I'm skeptical - if it was no problem to run without the rear compensators, then why did Peugeot bother fitting them?

 

Will have to check the direction, but mine is labelled open and close. So at the moment mine is in the fully closed position. Before I had a leak it was set at (starting from fully closed) 12 turns open (where fully open is 16 turns). I am running std brakes all round with a hydraulic handbrake kit (uses std handbrake not a standalone).

 

this is the one I have Bias valve

That one looks different to mine, in fact "open" and "closed" sounds more like a flow control valve like what Craig is describing... that would explain why your leak stops when you turn it to closed. Mine definitely doesn't have a fully closed position.

 

Now tempted to take it to bits to see what's inside... maybe just a valve inside sticking or something??