Brake Proportioning (Bias) Valves
Please Note -- Doing Stuff To Your Brakes May Cost You, Or Others, Their Life. Please Be Careful
Lotus did various configurations for proportioning the front and rear brake bias -- each rear brake line has it's own set of valves so if one side gets out of specification you can get weird braking behaviour particularly noticeable under hard braking. The purpose of these valves is to regulate the amount of pressure going to the front and rear brakes under the range of braking forces and ultimately to prevent the rear brakes from locking up before the fronts.
Originally Lotus used a 3/15 main bias valve in conjunction with an additional upstream valve for clenched butt, emergency braking to prevent the rears from locking up before the fronts. In later cars both valves were replaced with a single 2/15 valve. In the Lotus Service Notes it says that if any issues are found with the 3/15 valves (and additional high pressure valve) then both should be removed and replaced with a single 2/15 valve.
So what does 2/15 or 3/15 mean? The 15 refers to: up to 15 bar the brake pressure is the same to both front and rear. Above 15 bar the 2 or 3 number kicks in and rear brake pressure is essentially kept at 20%(2) or 30%(3) of the front brake pressure.
The 3 dual valve arrangements with 3/15 valves are below.
1. Pressure limiting valve at 70 bar upstream of the 3/15 valve.
2. Pressure limiting valve at 65 bar upstream of the 3/15 valve.
3. Pressure proportioning valve (70:1) upstream of the 3/15 valve. Reduces any increases above 70 bar by 70:1.
Replacing any of these dual valve systems should be done with a single 2/15 valve according to Lotus.
The problem is that these are either no longer available or 90-110 GBP if you can find them. So what to do? Back in 2002 Mark Hooper wrote an article about re-building the proportioning valves, see here  The only thing missing from Mark's article was the dimensions of the O-rings and the sealing washer. Whilst I don't know the sealing washer dimensions, it is not a moving part and according to Mark should be serviceable.
The dimensions of the 2 O-Rings were measured by BobB from a proportioning valve supplied by Specky:
Measured the two "O" rings and they are as follows
smaller one is 3.75 mm id and 1.8 mm cross section
larger one is 7.1 mm id and 1.8 mm cross section
Now if you want to replace the seals then PLEASE NOTE regular nitrile rubber should not be used as brake fluid will eat it. You should obtain EPDM O-rings or O-rings made from something known to work with DOT brake fluid.
The spring which produces the 15 part of 2/15 has dimensions as follows:
Spring rate 4.58 N/mm
16 gauge wire diameter
7 coils ground ends
If anyone knows the dimensions of the proportioning valve seal then please notify me: adw at saska dot co dot uk
--Saska 07:27, 2 June 2010 (UTC)