Nova rear discs locking up.

[TWAG]

I'm running rear discs on my nova, I have a bias valve fitted but even with it wound right down the rear is locking up.

any one got any suggestions on what I can do to stop this?

[TWAG]

Forgot to mention, I'm using a Corsa Gsi master cylinder and servo.

[Stuart]

That's pretty normal as the majority of bias valves don't do the job most people think they do.

the only real way round it is to fit a pedal box affair to put a huge mc on the rear and give it almost no pressure

[TWAG]

I really don't want to get in to fitting a pedal box as I've only just rebuilt the car. Looks like ill be going back to drums at this rate. It wasn't locking up before I rebuilt the car which is what is confusing me.

Is there a standard vauxhall restrictor I could fit at the master cylinder that might help?

[Spudly]

Sound pikey but crimp the pipes to the rear end, will add more restriction to the flow, maybe your old lines were gunked up and reducing flow naturally, with new pipes and fluid you now have no restriction and theyre working as they shouldve done?

[TWAG]

Crimping isn't an option as I have braided lines front to back. I'm sure I've heard before about modifying the rear pads to reduce the braking area.

[bazil]

If your running 4 stud wheels just go back to drums,

Keeping unstable disc setup on the rear of a turbo nova is insanity

[Stuart]

Want change how they lock up, area of pad is to do with longevity of the friction material.

cp put an air bubble in his rear lines once to reduce the braking back there, that was till he got a pedal box

[dgbnova#1]

There must be a better rear bias valve as should be able to almost lock them off complete that's how I had my hand brake, pedal to floor then lock valve so maby an alternative one is in order. Funny I had disks on most of min no bias and never had this problem with loads of fast driving

[bazil]

I went the pedal box route using a 0.7 for the rear which was mk2 astra gte discs and calipers, I tried the standard gsi setup at first with poor results, changed the valves to 1.2 ones with no results so changed the full setup.

Edit, that was in 2001, I didnt have the internet back then for advice or help lol

Edit again, I was looking at your build thread ( very impressed too ) I have to say the im with DG on the bias valve, I didnt realise the setup you had when I posted my earler reply.

[Stuart]

Depends n the make and type of bias valve... I think what is needed for a nova is a lock off valve as opposed to a pressure limiter which doesn't kick in low enough

[Edd]

I'd either

Put drums back on
Or fit a Nova master cylinder with the rear bias things

[Pistol Pete]

Odd. I have discs on mine. Running whatever MC came on the Sri. Never ever experienced locking. I gave it stick round castle combe too!

[TWAG]

Thanks all the advice, I might look at trying another bias valve first then go from there.

if all else fails ill go back to drums.

[bazil]

I take it your master cylinder is from a abs equiped corsa? Looks to have only 2 ports

[craig green]

I ran rear discs from a twin-port Cosa M/C with no problems, mine was a rotary Wilwood bias valve & it was mostly adjusted to front bias, but I experimented with winding the rears in & you could feel the effect of the rears over braking, so I was quite confident in its abilities when I had it adjusted where I wanted it.

Maybe review the bias valve, confirm the specs of the one you have & swap it out perhaps.

[TWAG]

I take it your master cylinder is from a abs equiped corsa? Looks to have only 2 ports

Yes that's right.

[bazil]

Yes that's right.

Cool, I will look closer at your thread, I like the idea of this setup.

[scott.parker]

Look up bias valves fitted to Cav/Calibra with rear discs, im sure you can replace the valve part that attaches to the mc, so it restricts it at that point first..?

[davidfox280585]

fors ka inline rear valves will do the trick and can be hid underneath

[johnd]

I have disc on mine I used nova gsi servo, corsa/tigra non abs master cylinder and mk2 astra rear pressure regulators never had any locking up

[Johnny A]

Bare in mind my setup is untested but I think it's worth throwing this in anyway:
As far as I understand the Nova and Corsa M/C's use two pistons on a central rod, in the case of a Corsa it's one outlet port per piston (where the pipes are split at the ABS pump) and on the Nova two outlet ports per piston.

Now in the case of the Corsa-B M/C in a Nova you have to split the ports in-line. IF the front port actuated the front brakes and the rear port actuated the rear brakes then an in-line restrictor would do absolutely nothing. Both M/C pistons are on a common rod which means the piston driving the rear brakes (which is restricted) has additional resistance in the line but this doesn't mean more pressure is sent to the front brakes it just means the brake pedal is heavier. This is because the front/rear circuits are isolated and the two M/C pistons are on a common rod (so one piston cannot move at a different rate to the other).

Assuming I understand the M/C design is as I have stated and my logic isn't flawed.... and also these in-line bias valves aren't totally ineffective as stated earlier there are three ways around the problem:
1: A pedal box with adjustable bias
2: Run two bias valves in parallel (one valve/line) - this option retains the two isolated circuits and the bias valves force pressure to the (front) brake line on that circuit
3: Connect the two ports/pistons together (this is what I did - see pic below) - this essentially removes the isolated circuits making the two pistons drive one big circuit, meaning you can run one line to the rear with a bias valve in-line and the restriction forces more pressure into the other lines (fronts):

https://fbcdn-sphotos-h-a.akamaihd.net/hphotos-ak-prn2/1468650_10153414188460214_1386969803_n.jpg

Option 3 has a downside though, the reason there is two isolated circuits in the first place is for safety - should have piston fail you only loose 50% of braking power. By removing the isolation if one piston fails you would loose most if not all braking power.

[Benn]

Craig, what have you changed while rebuilding? You ran disc before didnt you?

Ive never had mine lock up, max ive had is a little squrim when ive broke mega hard.

[TWAG]

Craig, what have you changed while rebuilding? You ran disc before didnt you?

Ive never had mine lock up, max ive had is a little squrim when ive broke mega hard.

I've changed from copper lines to fully braided lines and replaced the rear calipers for refurbished ones.

[TWAG]

I've been looking at the wilwood Combination Proportioning Valve, see link below.

http://www.wilwood.com/PDF/DataSheets/ds715.pdf

If I'm reading it correctly this adjusts the braking from front to back rather than just reducing the rear. Any thought on this? Have any of you used this before?

installations wise it going to mean running two new lines into the car which isn't the end of the world.

[Benn]

I've changed from copper lines to fully braided lines and replaced the rear calipers for refurbished ones.

Same piston size in the calipers? As if you've gone down in size that might be way....

[TWAG]

Yeah same piston size.

[Benn]

Braided hose the same bore? Bit off if that's all you've changed...

[BRoadGhost]

Put larger disks on the front.

Take some weight from the front of the car.

Put some weight at the back of the car.

[TWAG]

Put larger disks on the front.

Take some weight from the front of the car.

Put some weight at the back of the car.

I have 280mm wilwood on the front which are the biggest I'm going to fit under my wheels.

adding weight is going backwards and there isn't anything I can remove from the front that I havnt already.

[TWAG]

I've just had a thought, I've assumed that by winding the bias valve knob down (clockwise) would reduce the pressure to the rear.

looking at some other bias valves it appears that they show the knob Turning counter clockwise to reduce the pressure. It might be as simple as I've wound the bias valve the wrong way..... Fingers crossed.

[Benn]

I have 280mm wilwood on the front which are the biggest I'm going to fit under my wheels.

adding weight is going backwards and there isn't anything I can remove from the front that I havnt already.

Whats the braking link bar the locking up? As i've been thinking about a Corsa set up...

[TWAG]

Whats the braking link bar the locking up? As i've been thinking about a Corsa set up...

Yeah it's good, much more direct feel I think. Although its been a long time since I driven it without the Corsa set up. I think you should do it as it will make a massive difference to your engine bay! If your unsure about how to do any of it just give me a shout, you can always look at mine to help with where to drill the holes etc.

[TWAG]

Well I feel a bit stupid now, just wound the bias valve out and the rears have stoped locking up.

Thanks for all the input, I'm sure it will come in handy to others.

ive just got to sort out the engine running issues now. New thread to follow soon.

[BRoadGhost]

Who said anything about adding weight..? ;O

Mmm those bias knobs can be counter intuitive; like a gas regulator.