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1981 honda GL500
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Hey all, I recently purchased new clip on bars, grips, throttle and levers for my 1981 gl500. After installing all the bits, I was attempting to bleed the brakes (new brake lever had a reservoir and all). I noticed the bleeder valve on the left is stripped and rounded and the right calipers valve had been broken off. Rather than try to remove the broken and stripped valves, I was hoping to upgrade to newer calipers. I've typed in every combination of keywords to find out what size I'd need but can't find anything. Anyone have a link to a compatibility list for dual piston front calipers for the gl500?

TIA!
 

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If you have both left and right calipers then your GL500 is a former Interstate. Interstate discs are a different diameter than the GL500. In addition to calipers you may also need to change the caliper bracket.

CB900 Custom had dual piston calipers.

You could take your calipers to an automotive machine shop an have the bleeders drilled out.
 

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'84 CX650E that is evolving into a GL500
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AFAIK, original replacement calipers haven't been available for a long time and I've never heard of anything from the aftermarket that is a bolt on fit so I think I'd start with a trip to the best bike wrecker/salvage in your area (from what I can find online there are several).

Honda used the same basic caliper type with several piston diameters on a lot of bikes so as long as the brackets that attach them to the forks are OK you can use just about any matching pair from any model, the exceptions being '83 and later GoldWings (the linked brake setup uses different size pistons in the left and right calipers) and GoldWing Aspencades that have wider calipers because of the thicker vented discs.

Left and right calipers differ in the location of the bleeder screws (they aren't normally valves) and the size of the pistons is indicated by a number stamped or cast into the caliper (the one in the pic below has 30mm pistons). I believe the piston diameters have 2 mm increments and you probably wouldn't notice if you changed to a size bigger or smaller than original as long as you have the same size on both sides (if they are different the side with smaller pistons will wear out pads faster)
208369

You mentioned replacing the "lever" and then said "new brake lever had a reservoir and all". This indicates that you have replaced the master cylinder. Front master cylinders generally include a new lever but the actual lever is considered a wear part so they are commonly available separately.
When you squeeze the brake lever the master cylinder pumps some brake fluid into the brake line. Since the fluid is incompressible the same amount of fluid is forced into the calipers to make the pistons press the pads against the disc(s) to slow their rotation and thus slow/stop the bike.
The amount of movement required at the lever to press the pads against the disc(s) depends on the ratio of the cross sectional area of the master cylinder piston to the total area of all of the pistons in all of the calipers so you need the correct master cylinder for the number of calipers your bike has. Master cylinders for use with two calipers must displace twice the fluid for a given lever stroke than ones for use with one caliper. If you use a MC made for two calipers with only one caliper the lever will have about half of the travel it should have and modulation of the brake will be more difficult. If you use a MC made for one caliper on a bike with 2 calipers it will need twice the lever movement to apply the brake and in some cases the lever may reach the handlegrip before the brake is fully applied.
IIRC, the optimal sizes for master cylinder are 12mm or 12.7mm (= 1/2") for a single front caliper and 16mm or 15.8mm (= 5/8") for dual caliper.
Note that: 1) the area of a 5/8" diameter piston is almost exactly double the area of a 1/2" piston and 2) dual piston calipers have smaller pistons than single piston calipers so the total area of the pistons is about the same.

BTW: I hope you are replacing the brake lines at the same time. The original rubber ones were supposed to be replaced a long time ago but stainless braided lines are good for the life of the bike.
 
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