Residual Pressure Valves

[EuropaTC]

Hi folks,

As some of you know, I was struggling to improve the brake pedal feel after doing a rear disc conversion and was advised to look at fitting a residual pressure valve to the circuit.  This was alien technology to me, so I wandered off to find out a bit more about them and thought I'd post the findings here just in case anyone else is interested.  Of course, after having spent an hour googling for info I'm now a complete expert....      

Ok, it seems these things come in two flavors,  2psi and 10psi settings. For a disc system you have 2psi, for drum brakes you have 10psi because the additional pressure is needed to overcome the brake shoe retracting springs. If you have a mixed disc/drum set up then either you go with the 2psi or you fit both with the 10psi only acting on the drums. (the info I found says that 10psi causes too much drag with discs)

I've posted a picture with them fitted to my car, I need 2 because it's dual circuit and they are both 2psi valves. When you press the brake pedal the fluid flows without restriction to the calipers but when you release the pedal there is a small valve coming into play which keeps a 2psi pressure in the lines between the valve and caliper pistons. This residual 2psi ensures the pads stay in contact with the discs so that the next time you press the brake you get a quicker response.

So why would you need them ? 

It seems to depend on the layout you have.  If your master cylinder is below the level of the calipers/drums in theory the caliper pistons can retract more than they should and hence when you press the brake you have this initial pressure to build up before you get the onset of real braking pressure.   

In practice I think it's a bit more complex because it's the hydraulic head of the system that's the real thing, not just the master cylinder location.  Mainly because if you have a remote reservoir mounted higher in the car, then the resultant head pressure is going  to be greater ?  The other thing that came to mind was that if you have servo's mounted in the engine bay then the lines to the calipers/drums would see that head pressure rather than the lower pressure from the master cylinder.

So, do you need them ?  Well I'm sitting on the fence here with a "definitely maybe".     

In all honesty I don't know if they will be of any real benefit to a standard system, and if you have servo's mounted high in the engine bay then I suspect not.  If you remove the servo's and re-route your brake lines accordingly then I think they will be of benefit, especially if you have one of the m/cyl reservoirs mounted low down on the car.

On my installation I checked the drag on the wheels before and after fitting and there's no significant difference with a residual 2psi in the lines. The wheels still rotate freely but when you look the pads are dragging on the discs.  In my set-up the pedal still has more travel than I calculated it should have,  although it is less than without the valves and the pedal does feel firmer on the initial application.

Is that down to the 2psi or simply that I've bled the system better with all the practice I've been getting ? I don't know for certain, but the theory says it's the valves because with my reservoir position I can't get 2psi just from the hydraulic head.

Conclusions ? It's not a "gotta have one of those" things, but more of a solution to a specific problem you might find when re-designing your system.

Brian

[exarkun1178]

What is your evaluation of the disc conversion?  is it a worth while mod?

[EuropaTC]

is it a worth while mod?

Hmm. Now that's a tough one to advise because I expect that unless you produce a kit yourself then you'll be looking at $1000 for a bolt-on kit of parts. So I guess all I can do is list the plus/minus side of things because I think it's not a straightforward call.  So let's start off with what I've found as a plus;

1. The pedal, now that everything has bedded in, has a modern-car feel to it. I suspect less travel than previously but I wouldn't swear on that one.
2. The brakes work fine but I've no way of comparing disc/disc to disc/drum because I'm just not that good a driver. I have noticed it stops much quicker when backing out of my garage and the discs are "front", but in that scenario the drums are inefficient anyway.
3. In theory the OEM drums contribute 25% to stopping. With discs this increases to 33%, more if you fit larger rear discs. So there will be an efficiency improvement but in practice I wouldn't describe it as "jaw dropping"
4. My conversion (DIY) was cheap. In fact it was cheaper to fit the discs than completely overhaul the drums. IIRC somewhere around $200-$300 but I listed the costs on the original article.
5. The handbrake/park brake is excellent. It really is, it will lock the rear wheels which is something I never managed with the drums.

The minus points.

1. Non-original so will upset the concours guys
2. Because of my choice of disc/rotor the offset from the mounting flange meant that I could no longer fit my 13" Cosmic alloys. Other wheels may differ, steels should be ok for example.
3. Cost is a minus if you're buying a kit. Much cheaper to overhaul & fit new drums,etc.


I guess the real question is, do you really need any more rear braking ?  The period road tests all praise the Europa for brakes and I'd guess to make a significant improvement you'd improve tyre grip, front brakes and then the rear brakes if you really wanted to stop quicker.

I had different motives; our UK annual test measures the handbrake and it's always been marginal (even the period road tests said that) so I wanted to tick that box and a rear disc certainly does that.  I'd already got some larger alloys so more tyre grip all round and plenty of room to fit rear discs. And finally, I get a kick out of this sort of homemade modification, I like making things and solving the problems. (plus I had some help from another UK owner)

Would I have spent $1000+ to do the mod ? Nope, never in a million years.  I'd have struggled on with the OEM kit. 

YMMV as they say....

Brian