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The foot brake pedal on my 91 Aspy is soft. I bought this bike last spring. The pedal was okay but started getting lower and lower.

I took it to the dealership. They changed out thefluid and the pedal was fine for a couple rides then began to get lower and lower again.

I am now considering rebuilding the master cylinder.

Does this make sense? Anything else I could try? Where can I buy a rebuild kit?

Thanks in advance for any help.
 

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Is the fluid leaking out somewhere? Have you put it on the center stand and pumped up the pedal and held it? Has the battery leaked acid out, you would have corrosion under the battery on the frame, and if so, check the hard lines coming off the m/c, I had to replace one that was leaked on and caused a small leak.
 

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Bleeding down the foot-brake can be a bugger -- it's coupled to the left-front caliper, as you know.

The trick has always been to bleed the front caliper first, then re-bleed for the rear. -- Unless there's a leak in your M/C bypassing or not-building pressure, there's usually not much more than some bubble somewhere givin' you a hard time ... Pressure bleeding from the M/C is NOT recommended since the reservoir is remote from the M/C and the connecting hoses/clamps will let you know that they don't like to be pressurized (ask me how I know). A good vacuum bleeding should work. Clean DOT4 as always.

If you're still losing pressure after a solid flush/bleed fo both braking circuits and the lines and calipers are all dry and free of leaks, then the M/C may be your next stop. (personally, I hate the labor invoved with the rear M/C, so I'll try anything else first!)
 

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The master cylinder itself will usually leak down under steady and light foot pressure if a cup (rubber)is bad. With that said, there is a part called the residual check valve at the output of most master cylinders. It keep a few pounds of presssure on the calipers to keep the caliper pistons from retracting. If residual check valve fails the pistons might retract and it would require more pedal travel to fill the void before the brake pad contacts the rotor. (more brake pedal free play) In an ideal world the pad is against the rotor with a couple pounds of pressure. The second you add any fluid to the equation, by stepping on the brake, it is ready to respond.

I would think that either you have a problem with the resdidual check valve or a mechanical problem (Worn wheel bearings, warped rotor etc). Anything mechanical that moves the wheel or the caliper axially will tend to push the piston in. (retract the caliper) The residual check valve is usually a rubber and very light spring at the very output of the master.

Another thing to remember is that the pedal and piston must return all the way to the start of it's travel. That is the only place where it can reload with fluid. There is a hole (passage) from the reservoir to the piston bore. If the piston does not return fully the passage is blocked and the piston bore can not fill completely.

The more I write the more I have a decision that if it were my bike I would look over the mechanical end of the bike. If the rotors are OK wheel are tight axially, I would repair the master.
 

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Thanks for the ideas. Here is some additional info:

The bike was last ridden Nov. 2 (1.5 months ago). At that time there was very little braking on the foot pedal. Pumping the pedal increased the braking very little.

After posting this morning, I went out to the garage to check a few things. There now seems to be pretty good brakes on the pedal. When I hold the pedal down, it holds steady with out bleeding down. The pedal is lower than I would like, but definately has good braking. The fluid level is right at the top. There are no signs of any leaks. The battery and local area are very clean. I spun the rear wheel by hand with the bike on the stand. It sounds and feels like the pads are in contact with the disc. Guess I should do the same with the front.

I am the third owner of this bike. It is 19 years old, but only had 23,000 miles on it when I bought it this last spring. Obviously it has set for extended periods of time. It would appear the bike was always stored indoors. The engine is very clean. The wheels are dirty, but not oily or grimy.
 

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You know just something to try as someone here recommended it to me. Put one of those resover bleeders (you no the tube with the small tank on it so air can't get back in) Bleed the brakes normally. Then with the bleeder still attached put a sledge hammer on the brake pedal. Leave it there all night then in the morning give it a few pumps hold the pedal down and tighten everything back up. I got a large bubble out the next morning and haven't had any further problems with the system. It might not be your problem but I have heard others using that method and it sure worked for me.
 
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