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I have a '79 R80/7 with an under the tank Master Cylinder. The brake pressure light stays on all the time. Can the corresponding switch simply be bypassed?

That is a fluid level light. You may want to check the level.

► Robert is correct. The red warning lamp is a fluid level warning light, which is required since the M/C is under the fuel tank and out of sight.

These switches (located in the reservoir cap) go bad, what with corrosive DOT4 fluid splashing all over them. Typically the electrical contacts on the exterior of the cap fall off, which is probably why your lamp turned ON.

BMW wants about $50 for a new cap assembly. I found a NOS Ate reservoir cap on Ebay for $7 off some 70's German car. The only difference was the length of the stem for the float was much longer. But the way Ate designed these makes it easy to pop off the float, shorten the stem, and reattach the float. It took me all of 15 minutes with a pocket knife to make it work on my '79 R100.

► The brake pressure switch on the front end of the M/C is what activates the rear brake lights. I seriously doubt you want to unplug that and leave it in a disabled state. If that switch is bad, then unscrew it and get a new one.

Hope this helps.

Robert, I've got a couple of spare caps - I'd be happy to send you one at no cost. Flick me a PM if interested.

Wobbly wrote: ↑Mon Nov 20, 2017 10:30 pm ► Robert is correct. The red warning lamp is a fluid level warning light, which is required since the M/C is under the fuel tank and out of sight.

These switches (located in the reservoir cap) go bad, what with corrosive DOT4 fluid splashing all over them. Typically the electrical contacts on the exterior of the cap fall off, which is probably why your lamp turned ON.

BMW wants about $50 for a new cap assembly. I found a NOS Ate reservoir cap on Ebay for $7 off some 70's German car. The only difference was the length of the stem for the float was much longer. But the way Ate designed these makes it easy to pop off the float, shorten the stem, and reattach the float. It took me all of 15 minutes with a pocket knife to make it work on my '79 R100.

► The brake pressure switch on the front end of the M/C is what activates the rear brake lights. I seriously doubt you want to unplug that and leave it in a disabled state. If that switch is bad, then unscrew it and get a new one.

Hope this helps.

Ah ha!
Yes thanks. It was only a few years ago that I messed with the UTT M/C I had on my R100 but I seem to have forgotten everything about it .
So if I can recall that I forgot , its not dementia yet is it?

Airbear wrote: ↑Tue Nov 21, 2017 3:20 am Robert, I've got a couple of spare caps - I'd be happy to send you one at no cost. Flick me a PM if interested.

Oops - perhaps it's me that has dementia. It was dirtsurfer I wanted to offer the spare cap to. The offer is still open if you need a cap, ds.

dirtsurfer wrote: ↑Tue Nov 21, 2017 4:48 pm Yes thanks. It was only a few years ago that I messed with the UTT M/C I had on my R100 but I seem to have forgotten everything about it . So if I can recall that I forgot, its not dementia yet is it?

No it's not quite up to "Old Timer's" disease, merely an acute case of CRS (Can't Remember Stuff).


If you'd like to forget the care and maintenance of your under-tank M/C for the remainder of your life, then you'd do well to look at the advantages of DOT5 Silicone brake fluid. As long as the brake system is NOT an ABS system, DOT5 Silicone offers you several major advantages over DOT3 and DOT4 types....

• It does not attack plastic or painted surfaces
• It does not require annual replacement or bleeding
• It does not build up a varnish on parts internal to the M/C or caliper(s)
• It does not attack brass electrical connectors and relays found below the under-tank M/C models
• It does not absorb water or water vapor

DOT5 is the standard brake fluid on most Harley-Davidson motorcycles and was developed for racing in the 1970's, so for most street riders it will suffice. However, if cannot be mixed with either DOT3, DOT4, or DOT5.1 fluids. So the best time to convert your system is when you have a major brake system repair, such as M/C rebuild or brake hose replacement.

I have been using DOT5 Silicone on both my /7 Ate and single-sided Brembo brake systems for many years with no loss of brake function and zero brake system maintenance.

Just my 2 cents.

As far a Dot 5 brake fluid is concerned. Everything I have read on it suggests that not all seals are compatible, and all seals should be changed if converting. It's about brakes so, I would proceed with Dot 5 fluid conversion with extreme care. There are a lot of variables, even with brake line material.

I posed this question to Oak 30 years or so ago and he was of the same opinion as you ZM. Best to stay with BWM recommended fluid (dot 3/4). Too important a component to take a chance with.

Wookie