How do you test a brake pressure switch ?

The big buck one 349.00 in the Moss cat.?

you mean the piece that puts a light on the dash that says, "prepare to hit the wall in front of you cuz yer brakes have failed"?

Didn't know it was that much!. I think I'd simply let it be broken, so long as the juice was still getting where it needs to go...for that price. but that's for the whole assembly and there is a repair kit for the thing. The switch itself seems n/a, but rebuilding the thing with new innards might assure the juice goes where it should. Ad even a dead switch ought to be an effective plug. eh?<G>

as to how to test. I'd suspect that with a volt meter you could figure it out. there's either going to be voltage when it fails or there's going to be voltage all the time and failure breaks it. Look in Ashley's book...he might have that there. Might have to find another car to compare readings with. If I think of it next time I'm out to my car, and I have someone to step on the brake while I look at under the hood, I'll check what the "seems OK" condition is. but you've got several other guys up there who for a couple Watney's might be willing to participate.<G>

This is a multifunction device. 1) It tells you if the pressure on one circuit is greater than the other by way of a moving piston in the block on the fender. 2) Pushing the button inside on the dash should make the light glow. This is the test function. Neither function is a very reliable indicator of problems. I suggest ignoring it but if you MUST get it to work it will take some troubleshooting. Are you up to it? You will need a voltmeter

Voltage feeds the test switch via a purple wire attached to the bottom fuse (hot with key on). Check for voltage at the purple wire. If voltage is present, then move on to one of the wires hooked to the sensor on the valve body that your brake lines tie into. If you do not have voltage here, then go back and check the light bulb in the test switch. If you do have voltage at the sensor, ground that wire with a jumper and the dash light should come on. The sensor is not really a sensor at all; it is merely a contact that goes to ground inside the block when the piston in the block moves from its centered position. The test switch does the same thing when you press the button; it allows the circuit to go to ground, completing the circuit, making the light glow.

If the bulb is bad, it will not glow
If the test switch is not grounded (black wire) the light will not work
If someone has cut the bridge wire inside the connector at the block, the light will not work on "test" but may work when there is a hydraulic problem.

The switch itself is plastic and can be carefully unscrewed from the brass valve body without losing any pressure. You can get this part separately: Moss #141-725.

What exactly is the benefit of a brake failiure light/test switch? When you start the car
you're going to notice right away the brakes have failed - we all lurch out just a touch
before going anyways.

if it fails while you're driving, you'll notice it before you notice the red light.

Repair is as noted above.

Usually the o-rings in the slider fail causing fluid to get into the switch contact causing it to corrode. If your o-rings are leaking you need to either replace or install a suitable bolt and copper washer where the plunger mechanisim goes.

Thanks for the help I should have been more specific This is what I was asking about. I lost it to some one else.

Has anyone else had one that failed? They are BIG bucks!javascript:ol('http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem%26amp;item%3d330128747847%26amp;sspagename%3dADME:L:RTQ:CA:1');

PaulM Wrote:

I would have to say as well, you don't need it, it's a idiot light at best. They do the same thing on the Midget, the brass body is like over two bills, I've used these things off parts cars befroe to house a hydraulic brake light switch on race cars. In my opinion it's a switch/warning system you don't need, it's like a low oil pressure switch on a modern car, by the time it goes off, game over anyway. I know that doesn't answer the question, but I guess the point I'm trying to make is why bother, spend that money on something else.

Thanks Everyone.

Good point Hap. The reason I was thinking about the cheapy on EBay for 10.00 was the cost of a new one at 350.00.

Paul. The only thing that normally goes bad in the main brake pressure failure valve is the shuttle seals. Moss and others sell a seal kit for repair. This link shows the detail of the brake pressure failure valve.
http://www.panteraplace.com/page89.htm

The white plastic switch on the valve body will not prevent leaks if the shuttle seals leak. If you use a DOT 3/4 brake fluid the leaking fluid will eventually eat the bridge connector between the two pins in the plastic switch connector causing the circuit to stop working. The plastic switch is available from Moss, item 29 #141-725 in this plate.
http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=29029

You can check the white plastic switch with an ohmmeter, there should be no resistance between the connector pins. There will be no resistance between the either connector pin and the switch plunger in the deperssed position. To check the dash circuit disconnect the wire going to the white switch and connect the two wires together with a clip and push the dash light test switch. If the test switch works with the wires clipped together but does not work with the wires normally hooked uo the bridge connector in the white switch is open.

If you don't care about the system workinga good fix is to use a 3/8" fine thread short bolt to replace the plastic switch. Use a copper/plastic washer uinder the bolt head.

The above information applies to the dual brake circuit used in 68-74 MGB's.

Thanks Clifton

I agree, these valves are ridiculously priced if you buy new. One thing I'd like to point out if you're buying the rebuild kit (which consists of two pairs of slightly different o-rings and a copper washer); I have a leak in the valve off my '69 GT due to bad seals on the piston. No problem, I have new o-rings awaiting. Removed the piston from the valve and found out this particular unit uses a different seal set-up. Rather than o-rings, it uses "cupped rubber" seals at each end. As far as I can tell, these are not removable. This piston doesn't match the ones shown on the kit instructions.

Needless to say, a new piston with o-rings is on order. Not the end of the world but thought I'd pass along my discovery. My valve isn't the only one with this type of piston. I picked up another used one from a friend to see if I could make it work and this one has the same "cupped" seals installed. Must be unique to the late '60's cars when BL went to the dual-circuit brake system.