Lazy brake pressure switch – Flag not going up.

Don't start working on the hydraulic brake system until you have completely de-pressurised it. This takes dozens of foot brake pedal applications.

The pressure switches are mounted near the accumulators and look like this.

The one on the left was sticking when and only worked when the box on top was tapped with a hammer.

The pressure switches have a micro-switch in the square housing about the size of a matchbox.

Here are a couple of pressure switches from a scrap RM. The one on the left has two wires and operates the red light for gearbox air pressure. The one on the right is for the hydraulic brakes and has three wires. This unit was packed with grease but the micro-switch plunger was rusted and did not operate.

The micro-switch in the units were made by Burgess and don't seem to be readily available now.

I found the one on the right which is made by Cherry and you can see the model number is on the unit. It is a very close match to the Burgess unit on the left but the plunger is offset by about a millimetre. It does seems to match up with the actuator in the pressure switch. The body is slightly narrower but it can be packed out to fit snugly in the box. The hole centres are exactly 1 inch apart and the distance from the hole centres to the top of the plunger is also exactly 1 inch, the same as on the Burgess switch.

I found that the plunger on the Burgess micro-switch on my bus was slightly rusted and this is why it was sticking. I carefully cleaned the plunger and oiled it with a light oil and it worked fine.

I reassembled the pressure switch and refitted it to the bus. Magic – the flag came up after about 45 seconds. No tap required.

Now to test the hydraulic pump output and the brake warning switches. I did this by connecting the pressure gauges to the test points inside the A frame, close to the pressure switches and accumulators. Thanks to Rob Duker for lending me the gauges.

I followed the instructions in section 18 of the New RM Maintenance Manual by GL Walker and CM Walker.

I'm not going into detail here. If you are going to do it you should buy the manual. You won't regret it! By the way the Manual says you should do this test procedure every 12 weeks!

On starting the engine and letting it idle the pressure built up gradually within the time frame shown in the Manual indicating the pump output was satisfactory. The pressures reached the levels shown on the gauges and stayed at these levels. Fortunately the pressure switches operated the flag and the white light at the recommended settings and did not require adjustment.

My problem was caused by water getting into the micro-switch housing. The PVC insulation on the three wires had hardened and shrunk with age and were loose in the grommet. I took care to make sure the gasket at the base was sound and greased the wires where they pass through the grommet.

I'm not sure whether packing the inside with grease is a good idea. The unit from the scrap bus was packed with grease but was still very corroded.