Front brake pressure build up and not releasing

[dalai-lama]

I'm fixing a friends 1998 Valk that has been sitting for about 5 years.  The clutch damper plate went and he had another bike and never got around to working on it.  So I've done:

clutch rebuild
carbs replaces with a new clean set
desmogged
air filter
oil and filter
timing belts, stock trigger wheel installed, and I/S ICM
spark plugs
Pingle and Danmarc

For the brakes I rebuilt rear and front calipers as well as completely flushed the fluid.  I took it for a test ride and the brake pressure in the front built up and pretty much stopped the bike.  I cracked the one bleeder and fluid shot out and I was able to make it home but by that time, pressure was building again.

I ordered a master cylinder rebuild kit and did that yesterday, replacing the rubber seals, piston, spring and boot.   While I had the master cylinder off, I made sure it was completely clean and the best I can tell it is.

The brake lines were completely drained and I had a hell of a time to get pressure to build again.  I ended up using a 100ml syringe and filling the lines from the bleeders back up to the master cylinder and then manually bleeding the master cylinder and attached bango bolt and then did a normal bleeding.

So I took it for a test ride again tonight and just around the neighborhood and the pressure started to build again.  The pressure actually was building just letting the bike sit out in the sun, but after the little ride, I just made it back to the house.

This time I cracked the bango bolt at the master cylinder and fluid came out and the brake released.

So I am pretty confident that the calipers are ok as releasing the pressure releases the brakes.  I have released the pressure at the bleeders and that worked and now have released pressure at the master cylinder bango bolt and that also works.  So it seem that the brake lines should also be ok, given that releasing the pressure right at the master cylinder releases the brakes.  Also, I was able to push fluid from the bleeders back to the bango bolt at the master cylinder.

So the only thing that I can see is that the master cylinder is not allowing fluid back to it, thus causing the pressure to build up with a little heat and locking the system up.

I have not checked my 1999 I/S master cylinder but I don't remember if there is a return hole that I am missing or something like that.

Any ideas will be greatly appreciated.

[Bighead]

Head scratcher for sure and seems about the only thing it could be. Go on EBay and find a used one cheap and swap it out to make sure. If the bike has been sitting for as long as you say the owner shouldn't mind another weeks wait

[dalai-lama]

Looking at master cylinder images (from my easy chair), I think I might have missed this little hole (I know this is not the Valk master cylinder but it is very similar)



That is probably the return and it is probably completely plugged. 

[harbor]

Hola:  I'm no expert but there is a small check valve in the master cyl on the handlebar unit of every bike.  It often gets clogged  and creates the exact problems you list.  It is supposed to release the pressure in the lines and return the system to a ready state.  However, The valve in the master cyl does not release the pressure and so you have too much pressure in the line when you apply the handbrake. Problems then quickly ensue when you apply pressure fron the handbrake.   Cleaning the valve is the solution.  Or a new master cyl.  Do a google search on brake mastr cyl check valve.  This may not be a big problem,  and you can fix it yourself.  Good luck.

[Itinifni]

It sounds like the transfer port in the master cylinder is obstructed.

When applying the brake the piston first closes off the transfer port allowing pressure to build in the lines and calipers. When the brake is released to port opens allowing pressure to vent back into the master cylinder.
If the port is obstructed you will have the symptoms you described. As the fluid warms from the sun or the slight friction on the pads when riding it expands, if it can't vent back to the master cylinder the brakes drag. If they drag the fluid expands more...rinse and repeat.
It would also give you a hard time bleeding the brakes, if the port is obstructed no fluid will flow down to the piston making it impossible to build pressure and bleed the brakes. You performed a work around by injecting fluid up from the calipers.

The transfer port is nothing more than a strategically placed hole in the bottom of the master cylinder (like in the photo you posted). It may well be corroded and clogged. You should be able to clean it if that's the case but you should probably pull the piston back out to make sure you don't force any crap down into the bore that could hurt the seal.

I assume the piston and spring are ok since they're new.

Another possibility is the lever is not completely releasing. Is it an OEM lever? Is the pivot hole worn?
One way you can tell if it's the lever is to build some pressure as you have done (either by riding or letting it sit in the sun) then remove the lever. If the brakes release your transfer port is ok, if not you heading back into the master cylinder.

Good luck,
Matt

[dalai-lama]

I believe you are right Matt.  I will report back in the morning after I pull the master cylinder again and take it apart. 

It has been a while since I had worked on these as I tend to keep mine with fresh fluid and don't let them set for 5 years   I also was too lazy to pull apart the one on my I/S to take a look 

Funny after doing all the other work, this slowed me down

[Karen]

Brett, I can't believe that you actually had to ask somebody about something on a Valkyrie!  Do I now have to expect dogs will be sleeping with cats?    Is the end of the planet, as we know it, imminent?  How am I ever going to sleep at night again, knowing that you didn't have the answer for this one on the tip of your tongue?    BRETT KNOWS EVERYTHING!!!    AND DON'T ANY OF YOU EVER FORGET IT!!!!      Have a nice day.   

[dalai-lama]

Brett, I can't believe that you actually had to ask somebody about something on a Valkyrie!  Do I now have to expect dogs will be sleeping with cats?    Is the end of the planet, as we know it, imminent?  How am I ever going to sleep at night again, knowing that you didn't have the answer for this one on the tip of your tongue?    BRETT KNOWS EVERYTHING!!!    AND DON'T ANY OF YOU EVER FORGET IT!!!!      Have a nice day.   

   Funny Karen.  Today I get to try out your Digisync instead of my trusty twinmax.   I hope to get Joe's bike back to him this weekend.  Funny thing is that the deal was that my son Steven was going to do the work and the repayment would be my old Kawasaki Concours coming back to him from Joe.   With the Boy having no spare cash for registration and insurance, it looks like that might be coming back to me  instead.  It needs the steering head bearings changed.

[Karen]

It really never does end, does it? I'm sure you'll have both bikes in perfect condition very shortly, and maybe get to take something out for a joyride soon!  And here I thought that "The Boy" stories were a thing of the past... 

[TJ]

Had the same problem on a bike I bought... After rebuilding all parts ( M/C-Calipers ) 3 times
I replaced the rubber brake lines... Problem GONE....  Lines were closing when getting hot ( swelling shut on the inside )

[dalai-lama]

Problem solved.  The return hole was completely blocked.  Even with a pick, I could not break it free.  I had to drill it out, started with a 1/32 and went one size up from there.  Got it all bleed and took it for a ride and all it good!

Thanks for the help guys!

[dalai-lama]

Here is the return hole before I worked on it:


Here is after I drilled it out



In case anyone else has this problem in the future.   I swear when I looked and poked at the return hole it was like it was solid metal 

[Harryc]

Right idea but I am wondering if the enlarged return hole will effect the brake system pressure. The original return hole is much smaller. You method didn't seem too accurate ... just say'in. Personally I would not trust it.

[Bighead]

Right idea but I am wondering if the enlarged return hole will effect the brake system pressure. The original return hole is much smaller. You method didn't seem too accurate ... just say'in. Personally I would not trust it.

How can you tell from his picture the hole is larger?

[Attic Rat]

I usually use a carb jet reamer to clean those return holes out it works great. I had one on a clutch a few years back that just about drove me crazy till I figured it out. I am glad you got it repaired. Sorry I didn't read this post sooner I could have saved you a lot of trouble.

[Itinifni]

I wouldn't worry too much about the port being slightly larger, once the piston moves past the port the diameter of the port is irrelevant.

I'd be a little concerned about drilling because it may leave a small burr on the piston bore side which could damage the piston cup and it may take a little time to show up. The first symptom I'd expect to see would be the lever slowly dropping toward the grip under light brake pressure.

Using a torch tip cleaner works great for small passages to.

Glad you got it figured out.
Matt

[dalai-lama]

I polished the inside with 000 steel wool to remove any burs Matt.  I will pull it back apart in a while of usage and check however.

[Itinifni]

Since you cleaned up the bore after you were done no need to worry about it. Nice job!