1000SP brake valve

Hi, I’m going over the brakes on a 1979 1000SP that has been standing for a while. I would like to change the seals in the brake proportioning valve but haven’t managed to find anywhere that does them. Does anyone know if seals/a rebuild kit is available? Alternatively, does anyone have the specs for the seals and a recommendation for a specialist supplier to get them from? Thanks, Angus (Bristol, UK)

I have a similar age Spada and have replaced the master cylinder and caliper seals a couple of times in my 32 years of ownership, but never done the splitter unit.
I am not sure if spares are available for this. The parts list for the bike shows it as a complete unit (part number 18668560). I did buy a cheap unit off ebay to take apart to see just what is inside. The unit was rusted solid and siezed, yet the seals still appeared to be in very good condition. I would grease it (with the grease you get in the caliper kit) and reassemble it carefully.

Hi Don, thanks for your reply. Interesting that the proportioning valve should be listed as a single component in the spares list. I suspect that your plan to grease and rebuild the unit is probably the most realistic and will probably end up doing that. As I’m doing all the rest of the seals, I thought it would be good to do these ones too but would be reluctant to disturb them unless I know that I can get replacements. Cheers!

I recommissioned a Spada that had been un-used for 15 years. I stripped and cleaned the proportioning valve, because I had to repair a broken bleed nipple. Seals looked good and it seems to work OK.
They are very hard to find, so my back-up plan was to fit a plain splitter. I don’t think I ride hard enough to really need the “anti-skid” that the designed brake pressure proportioning is supposed to give. I suppose a further step could have been to fit the smaller calliper on the back, which I think was done on some other Guzzis.

That’s an interesting option. Is the main benefit of the proportioning valve (over a simple splitter) to limit excess pressure to the front caliper?

As far as I understand it, it actually limits pressure to the rear after a certain point, while still allowing more braking to the front. Supposedly to prevent rear lockup as weight transfers forward under braking. There are a few different versions, with different settings.
My Spada has a bigger calliper (P9?) on the rear, but I think some Tonti bikes have all 3 the same. The bolting positions are the same so it’s easy to fit a smaller (P8?) on the back, but I found the bigger one fouled the fork leg when I did a trial fit.
A plain splitter would give the same pressure to front and back.
I suspect my riding style never gets above the valve’s setting. I also have a loop with drum brakes so am used to looking ahead and not relying on hard braking.

If you remove the rubber cap of the valve, you can see the piston move up as you press the brake pedal. It does need to be pressed fairly firmly to make it work.
I have always taken the view that if it isn’t broke, then don’t fix it, but I totally agree that it would be nice to replace the seals as they are 42 years old.
I do have a spare on the shelf just in case I do ever need one.

The fluid in the rear system was pretty gungy and I wanted to strip it just to give it a good clean out. There was a small amount of corrosion on the piston and I suspect that the piston may have been stuck in he bore. It looks as though there is a small chunk out of the side of the outermost o-ring so it would be good to replace at least this one. I’ve contacted a few places that have been recommended without any success as it seems that the seals may not have been available separately and the valve is no longer available.

I think I’ll try and grease up the existing seals and see if there is any sign of leakage and if there’s still a problem will probably have to go down the route of installing a simple splitter. Like ranton_rambler, I probably won’t tend to brake hard enough to get the proportioning valve working but it’s nice to keep the bike original where possible and have the brakes as good as they can be.

I would imagine you should be able to buy a replacement O ring, they tend to be made in generic sizes. So long as the material is suitable for use with brake fluid. A google search suggests EPDM…
"Standard brake fluid compatible material is EPDM rubber (just like a radiator or heater hose). This is also a water-compatible material…but doesn’t like petroleum oils. "

Yes, thanks. I have ordered a couple of different sizes of EDPM o-rings so hopefully one of these should do the trick. I was interested to learn that Viton o-rings are not suitable for use with glycol based brake fluid (DOT 3,4 and 5.1) but interestingly some of the kits for replacement caliper seals offer Viton seals as an upgrade. Am I missing something?