Brake Pressure Limiting Valve

Brake balance on a car as light as a Mini is quite important, with so relatively little weight over the rear wheels it is easy for them to lock up if the brakes are applied hard, especially when there is weight transfer front to rear and or side to side. This can lead to unpredictable behaviour.

There are expensive solutions to this problem, twin master cylinders with brake balance bars and also expensive adjustable brake pressure limiting valves. However the solution I am writing about is one used a good few years ago by many a mini owner.

It involves making an adjustable brake pressure-limiting valve from a mini brake-proportioning device. It is very simple to do and extremely effective. In fact Aldon Automotive carries out exactly the same modification to the mini valve and sell it for around £60. My brake pressure limiter cost £1.10 and is functionally identical to devices costing up to £150.00.

Making the Valve

The first thing to do is to obtain a mini rear brake pressure-limiting valve, or brake proportioning valve as they are sometimes called. These are bolted onto the rear sub-frame of every mini on the front cross member of the sub-frame on the drivers’ side. The early cast type is the one to go for. If you prefer, a new one can be purchased for around £20. If you are using a valve from a scrap yard, ensure that you obtain the unions from the brake pipes that screw in as these are UNF. Note also that the flaring on the end of the brake pipefittings is different between the inlet (in the blunt end) and the two outlets on either side. One type (the inlet) is a single conventional convex flare; the other type (both outlets) is a concave double flare. If you are buying the valve new, make sure you buy two of the unions. In addition you will need a bleed nipple with a UNF thread.

If the valve is from the scrap yard, thoroughly clean the outside, removing all traces of rust and other shite. Strip the valve by unscrewing the large end cover, carefully remove all the internal parts and clean them thoroughly. Make sure you remove the plunger and spring.

If the rubbers look shot, obtain another valve, ensure that any varnish or other rubbish is removed from inside the valve, and that the threads for the unions and the end are clean. Drill the blind end using the drain hole as a pilot, then tap a thread to take an M10 or M12 bolt/setscrew, fit a suitable bolt and nut to help lock the position. Reassemble the valve using brake fluid as a lubricant. If you are using an M10 bolt you may need a small thin washer under the spring at the blind end for the bolt to engage on.

Fitting the Valve

The limiting valve needs to be fitted to the rear circuit of the brakes. It should be firmly mounted somewhere near the master cylinder where it can be easily reached  for adjustments to be made. The main feed pipe from the master cylinder to the rear brakes needs to be cut and connected to the feed into the valve. This is on the blunt end of the proportioning valve, the large end cap that unscrews. The other part of the main feed that now goes to the rear brakes should be fitted to either of the outlet connections. The other outlet should be blanked off with a UNF bleed nipple. Note that the ends to be flared on the brake pipes are different, one is a single convex flare and connects to the inlet that has a concave facing, the other is a double concave flare and connects to the outlet that has a convex facing. Make sure that you get this the right way round.

Setting Up

To adjust the front/rear balance you need to find an open space with little traffic and unscrew the adjustment bolt. Drive gently at around 20-30mph. Whilst turning declutch then brake gently. You should feel the front of the car run wide (understeer). Turn in the adjusting bolt around half a turn and repeat, not forgetting to declutch. Repeat this process until you feel the back of the car stepping out (oversteer). When this happens, lock up the adjusting bolt. Now drive in a straight line and make an emergency stop. The front wheels should lock up just before the rears.