Brake pump up

I have a Honda NX650 that basically just sits in my shop. Now the front brake has pumped itself to a solid lever that clamps the pads to the disc. I have already spent three days in the ICU due to this and need some help. I bled the line once and it felt fine. It sat for a couple more weeks and now it is pumped solid again. What is going on and how can I prevent/fix this? Naturally I am afraid to ride it and don't even want to let a prospective buyer ride it. Otherwise the bike is very excellent. Thanks.

number one cause is master cylinder piston is not fully retracting and exposing the bleed port that lets pressure from heating to escape to the reservoir.
rebuild or replacement is required.
a gummed up level could be not letting it come back.

possibility #2 is kinda rare but sometimes a hole in the inner liner of the brake line allows fluid to get between the layers and it can make the inner liner balloon and block flow.

my money is on the sticky master cylinder piston. it is a common problem.

Concur with Focus, plus I have seen the relief hole miss-machined on a couple MC. You can carefully elongate the hole and cure the problem. Usually it is caused by rubber parts that swell because of contamination of the brake fluid.
Can't blame you for being gun shy!
Smoky

Thanks, I appreciate all the help. I think its time for a full rebuild of the brake system after 27 years. This bike only has a little over 7k on it, way too much sitting. Everything is original.

I agree on the full rebuild. If you use some caution you might find that all the seals are OK and reusable. Over time the rubber line just needs to be replaced. I'm not saying blow out the pocket book and go with stainless, but if it is a rubber line, then it is perishable, its just science. While it is apart flush all the fluid out and refill with fresh fluid appropriate to what the manufacturer recomends. You might find that the seals, master cylinder piston, and the brake line are actually OK but if water has permeated your system then there is probably a large nasty gelatinous blob of goo in the caliper which is expanding and causing the piston to stick also. Any time you have an old bike, or a bike that is new to you, it's probably one of the safest things to do (as long as we're not getting into ABS territory). Fresh fluid will also go a long way on make those rubber lines last longer if they still seem serviceable but I'd replace the line with something a little more updated, just to be safe.

Good advice, thanks. I am very sure the fluid was still the original when I got the bike. I just could not figure what made the brakes pump up while just sitting. And then again after a good bleeding. Never had a bike do that before. I am afraid of riding on the street so I am selling it but do not want there to be any worries for the new owner.

Yeah a good friend of mine was rebuilding a Hawk (about 25 years old) and someone had upgrade the front brakes. It was a good upgrade but he went for a test drive after getting it running and the front locked on him. Fortunately he didn't do an endo or drop the bike but the pictures he showed me afterwards were eye opening.

Irishcoffee said:

Good advice, thanks. I am very sure the fluid was still the original when I got the bike. I just could not figure what made the brakes pump up while just sitting. And then again after a good bleeding. Never had a bike do that before. I am afraid of riding on the street so I am selling it but do not want there to be any worries for the new owner.

Click to expand...

What happens is when it is cool fluid pulls in but when the air temp starts to warm up the fluid expands but can't get past blockage so it pushes on the caliper Pistons applying the brake.
What commonly happens is you start riding and as heat transfers from the rotor to the caliper the brakes start to drag a bit heating up things more which causes more drag till the brakes finally lock up.
You can crack the bleeder and fluid will shoot out and the brakes will release but they will end up locking back up.

Would it be more correct to think if it is potentially water (or some similar contaminants that is affecting the boiling temperature of the brake fluid? I don't know what the limit is but I thought brake fluid was supposed to have a much higher boiling point than water, so it would stand to reason if there was water or a contaminant in it that it could drop the boiling point more, depending on how much moisture is in the system.

get the brakes hot enough and the water will boil displacing brake fluid and turning to steam.
depending on how much water is in the caliper (it is heavier than brake fluid and sinks to lowest level) you can get mushy brakes or a level that drops down till it is against the bar.
it also corrodes brake components.

a small amount of water is absorbed into the brake fluid, its over a extended time that more gets in beyond what the fluid can safely absorb.

it is wise to flush the brakes with fresh fluid every two years.

you can get test strips or actual electronic testers at auto parts places and test your fluid for water content.

Brake fluids are either glycol-ether (Dot 3, 4 and 5.1) or silicone based (Dot 5 only). All glycol ether fluids are hygroscopic, which means they will absorb moisture from the air at normal humidity levels. There are only two ways to combat this effect: A completely sealed system, or regular fluid changes.

Ignore your fluids at your peril.

Most manufacturers list brake fluid change as a regular part of maintenance. For most cars it's based on mileage, but there's also a time limitation o when it should be done. For motorcycles, it's wise to do it yearly.

Being hygroscopic is also the reason you see the "Use only from a new, sealed container" warnings. Over time, it will naturally absorb moisture - water - and if you use old brake fluid, you're only putting moisture directly into your system. Since you can get brake fluid in very small packages, perform a change, and recycle the rest with your other oils.

The effect described in post 1 is normally the result of seal fatigue. The seals (called primary or master seals) in the master have failed, and won't hold the pressure anymore, as evidenced by years of correct operation and the fact that you can "pump them up". A simple rebuild (and at this point, rebuild the entire system) will resolve the problem.

Thanks again for all the great, knowledgeable info folks. But one point that I am apparently not conveying well is that the brakes "pumped" up totally on their own while sitting peacefully in the corner of my shop. Last time the bike was ridden the brake performed perfectly as it should. Then a couple months later the lever was maxed out(solid-full). Zero movement. Wheel would still move but then locked up after less than half a mile. I noticed it immediately but my brain did not process what was occurring till too late. After a recovery period I bled the brake and things seemed normal with new fluid. Now after sitting for a few months the lever is SOLID again with ZERO movement possible. Bike has not been touched since the bleeding. This is not being caused from water being boiled while in use or anything similar. Zero use period. The crazy thing just pumps up solid while sitting there. Ghost in the Attic? Going to pull everything apart, rebuild and hope.

that's the master cylinder piston not fully retracting to the rest position.
the piston has to go fully back or it wont expose the relief port.
generally corrosion causes it but some adjustable levers can cause it.

the whole changing fluid thing springs from you saying it probably is original fluid.
that causes corrosion that leads to these problems.

I feel sure I was the first to remove the master cylinder cap. It wasn't pretty. I just can't understand how I could walk out of my shop with a fully functional brake and lever action ,,,,,then come back to a fully extended lever that will not move. Hope I can find all the oem parts I need.

Road gremlins!
When you left it working, was it cooler than it was when you returned to a hard lever?