Brake Warning Function

[Wheelz]

I have always assumed that the brake warning device fitted to the hydraulic system only performs the role of switching on a warning light, but does not affect braking in any way.

I have just heard the view that when the shuttle moves or is moved to one side, this restricts the flow in one half the braking circuit, which if true, could compromise brake function.

Any view on which is true and why?

[NeilR]

I would have said it can't, by design, move far enough to restrict any flow by a noticable amount. Just able to move far enough to activate the switch.

[marshman]

What Neil said. It would be a bad design if it did, it is purely a warning "device". Personally I think it would have been better to have a low fluid level warning lamp for the master cylinder reservoir as per modern cars.

I have just had a quick look at one and the ends of the shuttle do not come close to sealing off the orifice. It can only move a small distance before travel is stopped by the edge of the seal hitting the plunger on the switch. Also as we know in a braking system with properly adjusted brakes there is no real large flow of fluid, just enough to operate the pistons enough to bring the braking surfaces into contact. Finally the seals have a groove which is large enough to allow fluid through.

I suspect that the comments may have come from someone with a PDWA in their hand looking through the brake pipe connections and seeing the end of the valve apparently obstructing the passage without realising there was still clearance around the outside or that the valve travel is restricted by the switch plunger.

[Wheelz]

Thanks, that makes good sense to me.

[Philip Wardle]

Yes, all credit to Triumph for recognising that, as a general principle in system design, a secondary safety device must not introduce an additional failure mode to the primary function.