Master cylinder won’t bleed or build pressure

[Bubs]

Hello got a 1981 xs400. Trying to rebuild the front brake master cylinder. Got the kit took apart the master and the seals were destroyed all cracked and in pieces. So I don’t know where the new seals are suppose to go. Can someone tell me if this is the correct way everything is suppose to go in. When installed as shown it won’t bleed or build any pressure when depressing plunger with a screwdriver.

For reference It’s off the bike and I have the caliper hanging down off the table and the master on the table I’ve been filling the hose with fluid before I put the plunger in also.

 

[JPaganel]

That looks about right...

Maybe worth going back to basics.

Is the bore clean and smooth with no scratches? Are you sure you got the right seals? Did you make sure to clear out the small return hole?

 

[tstidham]

I had similar issues when I rebuilt mine last year. I bled mine while still mounted on the bike but I found tapping the body with a screwdriver handle caused bubbles to percolate out of the openings in the bottom of the reservoir. It took a while of slowly pumping and tapping to get fluid to start moving and reach the caliper.

 

[Bubs]

the kit came with the bike the plunger and spring is for sure correct. I didn’t have seals to base off of so unsure of those but they fit tight.

 

[Bubs]

That looks about right...

Maybe worth going back to basics.

Is the bore clean and smooth with no scratches? Are you sure you got the right seals? Did you make sure to clear out the small return hole?

The bore looks good return hole is clean it does bubble when pumping. Just doesn’t force fluid past. If I remove the plunger sling and seals I can run fluid through it.

 

[Bubs]

That looks about right...

Maybe worth going back to basics.

Is the bore clean and smooth with no scratches? Are you sure you got the right seals? Did you make sure to clear out the small return hole?

Wait by return you mean the hole under the small plastic flap ? I have not checked that one.

 

[drewpy]

good way is to back bleed using a syringe, pull the fluid down forcing it past the master seals. pump brake too to help move the fluid.
I've had mu callipers suspended above the master cylinder so bubbles rise to the bleed nipple overnight.

 

[kebabs]

Hello got a 1981 xs400. Trying to rebuild the front brake master cylinder. Got the kit took apart the master and the seals were destroyed all cracked and in pieces. So I don’t know where the new seals are suppose to go. Can someone tell me if this is the correct way everything is suppose to go in. When installed as shown it won’t bleed or build any pressure when depressing plunger with a screwdriver.

For reference It’s off the bike and I have the caliper hanging down off the table and the master on the table I’ve been filling the hose with fluid before I put the plunger in also.

if all seals are fine ,no leaks .keep bleeding took me 4 hours but got there.
I also use a piece of pipe with no return valve attached to the bleed nipple.

 

[Revvy Kevvy]

It took you four hours to bleed it? I have a similar issue and I have been pumping it for about 20 minutes....are you telling me it took you 4 hours of pumping??

 

[kebabs]

It took you four hours to bleed it? I have a similar issue and I have been pumping it for about 20 minutes....are you telling me it took you 4 hours of pumping??

pretty much so.just apply the lever nice and slowly..i would try cracking open the bolt where it meets the caliper and let gravity do its thing.the brake fluid will come through a lot quicker. or leave the reservoir cap off.attach a rubber band around the lever and leave it overnight. there's lots of different methods.

 

[Revvy Kevvy]

Ok, I will give this a shot tonight and I also saw that there is a reverse bleed method that apparently works. Its very different to the car master cylinder. I need to find some 3/16 silicon tube.

 

[kebabs]

Ok, I will give this a shot tonight and I also saw that there is a reverse bleed method that apparently works. Its very different to the car master cylinder. I need to find some 3/16 silicon tube.

yes it works to .fill your syringe. attach to the bleed nipple,it'll bleed through the master cylinder. do it slow.let us know how you get on

 

[JPaganel]

reverse bleed method that apparently works

Yes, it does work pretty good. I buy horse-sized syringes at farm supply stores for this.

I need to find some 3/16 silicon tube.

Doesn't have to be silicone. You can use the cheapest vinyl tubing for it.

The big difference is that silicone is effectively eternal, and vinyl will likely dry out and crack by the time you get to bleeding the brakes on this bike again.

 

[Revvy Kevvy]

Ok, tomorrow I go looking for tubing and a syringe but in the mean time I try the hold the brake handle all night idea.

Fingers crossed.

 

[kebabs]

Ok, tomorrow I go looking for tubing and a syringe but in the mean time I try the hold the brake handle all night idea.

Fingers crossed.

yip give it a go.or you can crack the brake pipe where it attaches to the caliper, pump the lever slowly till the fluid starts coming out then tighten the nut .u won't be far away with the bleeding process.

 

[kebabs]

yip give it a go.or you can crack the brake pipe where it attaches to the caliper, pump the lever slowly till the fluid starts coming out then tighten the nut .u won't be far away with the bleeding process.

how are you getting on with those brakes.are you still pumping

 

[Revvy Kevvy]

Every few hours I wandered into the garage and gave the pedal 30 minutes or so of squeezing and watched the air bubbles come out. I did this until it became nice and firm and thats how it is now. I remembered I had a brake bleeder I had bought to remove the oil on diff pump on a cyclotron years ago so I used that to suck through a few more tablespoons and now it feels really good. I observed that lots and lots of really tiny bubbles came via the tiny hole under the plastic tab so I am really glad I gave that a good clean out.

Now I need to reattach the rear wheel and roll it out of the garage to try and tune up the engine. I cannot tune it inside as the fumes make me feel sick after 30 minutes or so.

Pictures below of how it was bleed and how the bubbles looked. Expect it to take a full night and a fantastic thank you to all who provided advice from the forum, I was absolutely baffled when it would not pump as I am used to car ones that a few pumps of the pedal and volia ....fluid at the other end!!

 

[kebabs]

how are you getting on with those brakes.are you still pumping

Great u got therejeez its a wonder your not dead .30 minutes running in the garage

 

[Revvy Kevvy]

Yea, it was a bad idea. I got a terrible headache and a cough from my efforts of trying to tune a super rich engine in a small area.

I dont recommend it in future

Thanks for your help, I more worked on Cars and cyclotrons my life not really motorcycles although I have had a few. I guess I got old, found a hobby and bought a abandoned motorcycle

 

[kebabs]

Yea, it was a bad idea. I got a terrible headache and a cough from my efforts of trying to tune a super rich engine in a small area.

I dont recommend it in future

Thanks for your help, I more worked on Cars and cyclotrons my life not really motorcycles although I have had a few. I guess I got old, found a hobby and bought a abandoned motorcycle

must be an age thing