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View Full Version : How to test a Brake Pressure Failure Switch



ScottFromNH
05-29-2012, 06:32 PM
I have a 1973 spitfire that sat in a shed for 10 years before I bought it last summer.

I am trying to figure out how I can test my Pressure Failure Switch to make sure its working or if I need to buy a new one. Does anyone have any ideas of how this can be down?

thanks

scott

DNK
05-29-2012, 07:09 PM
Is it like the 6's?

TR3driver
05-29-2012, 07:43 PM
Are you wanting to test just the electrical switch, or the mechanism that actuates it?

Easiest way to actuate the mechanism (for an end-to-end test) is to have someone step on the brake pedal while someone else loosens a brake bleed valve. Turn the key on (no need to start the engine), press and hold the pedal, bleed until no more pressure. The light should be on. If you really want a detailed test, release the pedal, top up the reservoir, then repeat the procedure using a bleed valve at the other end of the car.

Geo Hahn
05-29-2012, 08:19 PM
...Easiest way to actuate the mechanism (for an end-to-end test) is to have someone step on the brake pedal while someone else loosens a brake bleed valve. Turn the key on (no need to start the engine), press and hold the pedal, bleed until no more pressure. The light should be on...

I'm not following that. Once the pressure is gone I would have thought the brake lights would be off.

TR3driver
05-29-2012, 08:35 PM
I'm not following that. Once the pressure is gone I would have thought the brake lights would be off.
This is a dual circuit brake system, and we are talking about the "Pressure Differential Warning Actuator", which lights a big red light on the dash (not the regular brake lights). The PDWA compares the pressure between the front and rear brake circuits, and lights the big red light if the pressure differential goes high enough to move the shuttle inside the PDWA. By bleeding at only one slave, the pressure on that side goes to zero, but the pressure on the other side does not.

I probably should have mentioned "firm" pressure, like trying to stop the car. It doesn't take a lot, but some.

Lots of folks assume they would know if half the brakes were not working, but it can be harder to tell than you think. When I picked up my second Stag (which has a PDWA that has been disconnected), the rear brakes were not working and apparently had been that way for longer than the PO had owned the car.

I didn't realize the problem myself until I did a brake test ... which resulted in the rear brakes finally moving and sticking on! Spent the better part of 6 hours in a Walmart parking lot, trying to get them freed up using only tools available from Walmart.

Geo Hahn
05-29-2012, 08:39 PM
Thanks! Was having a dense moment there.

dklawson
05-30-2012, 07:04 AM
+1 on Randall's comments.

If the Spit sat in a shed for 10+ years you can count on the PDWA (and probably the master cylinder) being full of crystalline gunk that has probably locked it up. If the PDWA seals leaked before the car was parked, count on the electrical switch being a bit gummed up also.

The archive pages of the Buckeye Triumph web site go trough the best explanation of how to service and recondition a PDWA. Seal kits are hard to find so the common fix is to rebuild the unit using o-rings made of EPDM (do not use "regular" Buna-N o-rings).

See
https://www.buckeyetriumphs.org/technical/Brakes/MCPDWA/MC.htm
and
https://www.buckeyetriumphs.org/technical/Brakes/PDWA/PDWA.htm

ScottFromNH
05-30-2012, 07:22 AM
Thanks Everyone...

dklawson, Yeah I have already replaced the master cylinder, brake lines, calipers, and wheel cylinders. Thanks for the links I will read up on it..

TR3Driver, Thanks for the help and I will try that test on both front and rear brakes to make sure it is working correctly.