New Front Master: An Experiment

Mileage: 29,451

Maintenance Summary:

• Installed New Front Brake Master Cylinder (14mm)
  • Replaced fluid  Prestone DOT-4
  • Bled brake

Having only one other, even older motorcycle to compare to, I hadn’t really had any concerns about the front brake on this bike.   It seemed to work just fine, that is until I began riding a more modern bike, one actually built in this century.  Now, the front brake feels stiff and wooden — almost broken.

The master cylinder that came with the bike is not original equipment so the question in my mind was, did the person who installed this select a suitable part with the correct cylinder size?   Lacking in any markings to indicate cylinder size I can’t answer that without taking the thing apart.  Rather than do that, I decided to install a new master cylinder known to have the correct cylinder size and see if the braking is improved.

One of the reasons that someone might install the wrong part is that the Honda repair manual appears to have a typo.   In the table, a smaller master cylinder size is specified for the GL500I Interstate model with twin rotors, while a larger cylinder size is specified for the single-rotor GL500.  The reason that this table is suspect is that the calipers for two rotors should require more brake fluid volume from the master, not less.

This bike is the non-interstate model with a single rotor, so the 14mm size would seem more appropriate. Even if the table is correct, going from a 15.87 mm cylinder to 14mm — if that’s what I’m doing — should result in a lighter brake feeling (i.e. more hydraulic leverage).  That said, the front caliper has two 30mm pistons for a total piston surface area of 2xPI*(30/2)^2 = 1413 mm^2.   A 14mm master has a surface area of 153.86 mm^2 giving a caliper-to-master ratio of 9.2.  That’s not a lot of leverage compared to a modern bike.  For a 12:1 ratio, we’d want a master cylinder size of 12.2 mm.  So, if this experiment fails to improve the braking,  I think I may try and find a 12-13mm master.

There is an argument to be made for requiring higher lever pressure (four finger braking instead of two).  In a panic, you’re less likely to lock up the brakes.

The Honda master cylinder with the angled reservoir is no longer available;  if it had been I would have bought that part.  In my search for a new master with a 14mm cylinder, 7/8″ handlebar, mirror mount and integral brake switch I seemed to have three choices:  really cheap ($18 eBay from China,  more expensive but no indication of better quality than the really cheap option, and very expensive ($140 Shindy).  As this is a bit of an experiment, I went with the $18 option as the part looked very much like the part that was on the bike.

While this new part looks nearly identical, it’s not.  In fact it looks even less refined than the cheap master that’s on the bike.  Here’s what it looks like installed.  If you look closely, you’ll see that the right cover screw isn’t flush.  That’s because the hole spacing in the lid doesn’t match the hole spacing on the main body.   Fortunately there doesn’t appear to be any operational problems with the cylinder and piston.  Unlike the original Honda master with an angled reservoir, the reservoir cover on this part isn’t level to the ground when the wheel is centered, so I’m relying on the cover’s seal more than I would like to keep fluid in the reservoir.

Service Hint:  Put the bike on the center stand and turn the wheel to the full left stop before opening the reservoir; it’s more level in that position allowing you to get more fluid in.  It’s also less likely to spill when opening, and it’s further away from the paint on the gas tank if you do spill.

If this master works better than the old one then perhaps I’ll eventually buy one of these spendy-Shindy model 17-652 ( 17-652B for black) master cylinders.  With the separated fluid reservoir and adjustable bracket, it should be possible to obtain a better fit relative to the angle of the handlebars.  Note that the Shindy has the required integral brake switch and mirror mount.

Test ride results: No improvement, unfortunately.