GM Proportioning Valves investigation for compatibility

[bobbrown]

The spring size and rate I posted was to convert a 3/xx valve to 3/15
The small O ring is the same on both the 2/15 and the 3/15 valves but the larger one will be different on a 2/15 valve as the piston size is larger and the springs will be different between the 3/15 and the 2/15 valves.

2/15 valve
Spring rate 7.11 N/mm
33mm overall length
14.5 mm od
11.5 mm id
wire thickness 1.94 mm
7 coils ground ends
Little piston 7 mm
Big piston 12.83 mm

3/15 valve
Spring rate 4.58 N/mm
33mm long
14.1 od
10.9 id
1.63 mm
7 coils ground ends
Little piston 7 mm
Big piston 10.87 mm

Note: You need to take distance the pistons travel into account when comparing springs hence springs are rated per N/mm in this case
The above are not 100% accurate as small internal dimensions are tricky at the best of times and the small piston or rather the cylinder is not accessible with out some special measuring equipment and it is the cylinder diameter not the piston that the force acts on as the O ring takes up the clearances.

The line pressures are not the full story when comparing the 2/15 and the 3/15 valves you have to look at the system as a whole, in fact at 90 bar the 3/15 gives you 2.5% more force at the contact patch of the rear wheels than the 2/15 valve. If the front brakes have been upgraded or just the pad material changed then this will affect the brake balance, both brake upgrades and a higher pad coefficient of friction will move the bias forward so more braking on the rear is desirable to restore the status quo.
Also it should be noted that with any of the above upgrades you are going to lock the fronts at a lower line pressure than the stock brakes and if you have stock front brakes that are not in very good condition then you could have line pressures higher than is reasonably acceptable (70bar). Most brake system are designed with 70 bar as the upper line pressure needed but needless to say can work well above that, as the safety factor will be huge.
It is usual for manufacturers to bias the brakes forward as this is the safer option and although not ideal it does mean that it reduces the risk of locking the rears before the front.
In an ideal world you would want the brakes on all 4 wheels to be capable of apply the maximum force with out locking for a given line pressure at the same time, which means at the point of locking but not locked, this is usually taken as 5% slip, this has to be done for various road surfaces, tyres and the most critical weight, a fully laden car will have a different weight distribution than one with just a light weight driver and an almost empty fuel tank and you have take weight transfer into account, so as with all things it is a compromise. The brakes will be setup such that in the worst case scenario the rears do not lock before the fronts and in ideal conditions this is going to be to the detriment of the stopping distance. Big front brakes do not mean you stop quicker in fact they can mean your stopping distance is further; the biggest advantage of big front brakes it they handle the heat generated from braking better.

Readings from the MOT are about as much use as an ash tray on a motor cycle, there are too many variables to make decent comparisons, like tyres, state of the rollers being used for the test, condition of the discs, type of pads, and the weight, add 30kg to the boot you will change the figures as will the weight of the guy testing the car.
The MOT brake test is only there to ensure the brakes meet the minimum standard, they do not look to see if the brakes are 100% and just as an additional point if you have a car with permanent 4 wheel drive or some types of LSD then the brakes can not be tested on the rollers and must be tested using a "Tapley" meter.
As a very very rough guide the reading give an indication but if they were too far out the car would fail the MOT, there is know way of measuring the balance front to rear on the rollers at a MOT station, it could be done if you had a pressure gauge attached to the brake line and could apply the same line pressure when testing front and rear brakes and you would need to know what figures you would be looking for at any given line pressure.
Also there is a tolerance on these valves and in the case of the 2/15 valve the line pressures can vary from 26.5 bar to 31.5 bar at an input of 85 bar and still be with in tolerance although optimal would be 29 bar. (85-15 * 0.2 + 15).

As a final point if “O” rings and springs are being sourced it is imperative that the “O” rings are the right size not just ones that seem to fit, as a slightly larger “O” ring will go in but will add stiction to valve, a slightly smaller one will mean they leak and you are not going to see the leak until the spring chamber is full of fluid; also the springs need to be the correct size and rate. If it was me I would send them off to get them matched to the original not just close, the same even if it meant they had to be made as specials.

[RayD]

It’s only on the second time of reading I realise Jackonicko wasn’t struggling with the complex figures associated with the proportioning valves, but rather with the simple MOT test figures

I'm puzzled at all the figures being bandied around - unless you can simultaneously measure the effectiveness fronts and rears, don't things like tyres and condition of the drum on the machine make such measurements of brake balance at least slightly problematic?

Sorry Jon ... but what brought that on I wonder? It gets mentioned again in Bob’s post.

Readings from the MOT are about as much use as an ash tray on a motor cycle ...

If this is meant to get one over on me you need to choose a better example, and try not to follow it up with ...

As a very very rough guide the readings give an indication ...

I don’t know that anyone has said they give more than a rough guide. At our test station they are still using a water gauge ... how rough can it get?

If people just take for granted everything you say Bob, without question, they won’t benefit from the occasions you've been put right.

Try to let everyone have their say, without taking offence.

Jon,
In the absence of anything better, forget about the condition of the rollers, tyres, discs, pads, weight in the boot, amount of petrol in the tank, weight of the operator, amount of pedal pressure, weather conditions, or any other variable ... things will all be fairly constant between the front and rear test and they will give a rough indication of the balance between the front and back brakes. If the brakes are otherwise working as they should they’ll indicate if the proportioning valves are working or if someone had fitted an adjustable set and got them widely out. They might even have recorded the difference between the types of proportioning valves but I realise it would be a bit of a long shot with them being so closely related.

Ray

[Jackonicko]

Ray,

In view of the 'roughness' of the figures, the fact that you're not simultaneously measuring front and back, I just didn't see the point of the detailed figures of what happened to all of those cars. Still don't. Yes, an MoT brake test gives a rough indication, I see that. But you lost me with the detail.

My primary reason for posting, though, was a concern that Bob Brown seemed to have got sufficiently fed up to have dropped out of this, which I thought a shame, in view of his credentials and proven expertise when it comes to brakes.

I guess I should have asked him all about it when I saw him last week, but I'm not very bright so didn't think of it.

[RayD]

Thanks Jon,

You might get a better understanding of the figures by reading this from 24 Jul 2009, 15:26 onwards.

Ray

[RichyP]

Saska, the seal dimensions you requested are 11.5mm I.D 2.36mm thickness and 3.12mm height. I've got some photos but after spending the last hour trying to get the things to fit on the page - I gave up!!

Just to add, Scotts new valves the 3/45's had a different large piston diameter from his existing 2/15's so the valves we put together are 3/45 pistons with 2/15 springs. What this equates to is anyones guess but with the weaker spring should activate sooner rather than later thus not over braking... I think!

Reason I took mine apart is even with rebuilt standard rear callipers - new internals, I wasn't getting enough rear braking for the MOT. (16" willwoods on front). Can't see anything wrong with the internals, but on reassembly there is an improvement - perhaps I need a 3/15 to bring the bias rearwards?

Rich

[bobbrown]

Just to add, Scotts new valves the 3/45's had a different large piston diameter from his existing 2/15's so the valves we put together are 3/45 pistons with 2/15 springs. What this equates to is anyones guess but with the weaker spring should activate sooner rather than later thus not over braking... I think!

Rich

The spring in the 2/15 valves is stiffer than the ones in the 3/15 valves so they will not begin to work till the pressure has exceed the 15bar by what amount I'll let some one else work out as all the information has been posted and I think the movement of the valve is about 5mm.

[Coupeboy]

Bob

Would you please advise if you have refurbished the 2/15 valve I sent you 3 months ago, and as promised, as I have not heard anything from you (not sure if my PM's have reached you) or received it back.

Thanks

Alistair

[muley]

Holy thread resurrection, Batman!!

Recently 3/45 and 3/35 brake proportioning valves have popped up on ebay. I have invested in a pair.

To (easily) sail through the MOT I removed both my standard valves and after a massive struggle dismantled them. One was clean and perfect inside as you'd expect on a 45K mile car. The other was full of crud in the spring chamber and the spring has corroded and the spring chamber was scored ~ I suspect the spring was jamming and stopping the piston from moving smoothly if at all.

Working on the basis that a bad BPV is better than no BPV (where have I heard that phraseology) I am going to modify my 3/45 valves to 3/15 ish valves by fitting springs from Flexosprings that are a very close match to the specs above.

The extra, minor step is that the NOS Vauxhall valves require an SAE brake flare on the output side, but these can be created with the cheapo brake flare tool..

I'll update soon.

Thanks are due to previous posters -:

Jim