Front caliper seized, how to rebuild master cylinder?

[KennyFSU]

I had the caliper rebuilt by a shop recently and the front brake was working fine until yesterday when it seized up again. Shop says the master cylinder needs to be rebuilt now so I bought a rebuild kit and plan on doing it myself.

I've seen a few YouTube videos on other bikes and it seems pretty straight-forward but I was wondering if anyone had any other tips or resources they use to rebuild one for a '91 Intruder 750, thanks all.

[navigator]

It's very straightforward.
Remove the fluid and banjo bolt, then unbolt the cylinder from the bars.
small plastic caps cover the allen bolts.
You may want to mark the bars and MC with a sharpie, to get it back in the same position.
Remove the snap ring with snap ring pliers, then slide the guts out.
clean the bore thoroughly, assemble the seals on the new guts and slide it back together.

[KennyFSU]

It's very straightforward.
Remove the fluid and banjo bolt, then unbolt the cylinder from the bars.
small plastic caps cover the allen bolts.
You may want to mark the bars and MC with a sharpie, to get it back in the same position.
Remove the snap ring with snap ring pliers, then slide the guts out.
clean the bore thoroughly, assemble the seals on the new guts and slide it back together.

Sounds simple enough, thanks again for the steps! [emoji106]

[Herb]

Don't forget to clean the reservoir and passageways really well.

[KennyFSU]

Thanks for the tips.

The front brake seems to work fine when left alone to cool down. After riding it for a few minutes and getting it warmed up it will seize on me again, any ideas??

[navigator]

Thanks for the tips.

The front brake seems to work fine when left alone to cool down. After riding it for a few minutes and getting it warmed up it will seize on me again, any ideas??

Use new, clean DOT4 brake fluid.

[KennyFSU]

Just rebuilt the MC, brand new brake line, and fresh dot3.

Only issue now is I can’t seem to bleed the brakes. Opened the mc cap, put a clear tube on the caliper nipple, several squeezes of the lever, hold, loosen nipple and allow fluid and air to escape. Repeated this process several times and not much air escaped and the lever still has no resistance.

I even bled the mc by removing the banjo bolt and placing my finger over the hold and pumped the lever.

Any ideas?

[navigator]

Remove the bleeder nipple and clear it out with a piece of wire.
Also poke the wire into the caliper.

[KennyFSU]

I'm at my wits end here.

Even with the caliper nipple closed and full brake fluid in the mc I cannot build up any pressure. When trying to bleed the system I did notice some dark swirls starting to appear in the mc (did not see any bubbles or squirting of brake fluid when pumping the lever though).

I can see a small amount of fluid/air coming out of the caliper nipple hose but nothing significant.

[navigator]

I'm starting to wonder if the seals in the master cylinder are installed in the correct direction.
The open or bowl side of both cup seals should face towards the pressure side, that is towards the spring.

[KennyFSU]

I'm starting to wonder if the seals in the master cylinder are installed in the correct direction.
The open or bowl side of both cup seals should face towards the pressure side, that is towards the spring.

I’m glad you brought that up because I was thinking about that as well but I’m 99% sure they are both facing the same way you described. I’m going to try bleeding just the mc again.

[KennyFSU]

I'm starting to wonder if the seals in the master cylinder are installed in the correct direction.
The open or bowl side of both cup seals should face towards the pressure side, that is towards the spring.

Tried reverse bleeding today and found some interesting info. The mc is actually blocking fluid from filling the reservoir (I unscrewed the brake line and was able to reverse bleed it right to the union bolt).
So I took my mc apart again, do the cups look right? One doesn’t...

[navigator]

According to the service manual, the cups are installed correctly.

[navigator]

If reverse bleeding does not fill the master cylinder, you have a blockage somewhere.
When you release the lever, fluid should be able to return to the MC. if it can't return, it is holding the caliper pistons in, which makes the caliper seem seized.
If this blockage is not in the small passages in the MC, and not in the (new) line, it has to be in the caliper...most likely where the line inputs into it.

[KennyFSU]

Sorry for the late replies, was traveling for work for the past 2 weeks so the bike just sat there but now I'm ready to tackle this issue again.

To recap: caliper was rebuilt and new pads installed by a shop. I rebuilt the master cylinder using a kit, replaced the brake line, and tried bleeding the fluid.

I can bleed the mc separately, I can bleed via the caliper only if I unbolt the brake line at the mc. Otherwise, once everything in buttoned back up I can no longer bleed the system (usual way nor reverse bleed).

The issue seems to stem from the mc but I have no clue what else I can do now.

[Herb]

What you are saying is that you can bleed all the way to the caliper by removing the hose but can't seem to bleed the caliper itself. Sounds like something is in the caliper preventing the fluid from pushing the air out.
I would try pulling the caliper off, taking it apart, and cleaning everything well. New seals for the caliper are not expensive and you are down to the only part left.

[KennyFSU]

What you are saying is that you can bleed all the way to the caliper by removing the hose but can't seem to bleed the caliper itself. Sounds like something is in the caliper preventing the fluid from pushing the air out.
I would try pulling the caliper off, taking it apart, and cleaning everything well. New seals for the caliper are not expensive and you are down to the only part left.

When I try bleeding the brake I can't seem to build up any pressure in the lever. When I reverse bleed I can't force any fluid into the caliper at all (using a giant syringe) BUT once I disconnect the brake hose at the MC I can reverse bleed the fluid right up to the banjo bolt. This says there is something clogged in the mc that won't allow fluid in.

Weird thing is, I can bleed just the mc (no hose) just fine, pressure builds up when I cover the hole with my thumb and even fluid squirts out.

[Herb]

What you are saying is that you can bleed all the way to the caliper by removing the hose but can't seem to bleed the caliper itself. Sounds like something is in the caliper preventing the fluid from pushing the air out.
I would try pulling the caliper off, taking it apart, and cleaning everything well. New seals for the caliper are not expensive and you are down to the only part left.

When I try bleeding the brake I can't seem to build up any pressure in the lever. When I reverse bleed I can't force any fluid into the caliper at all (using a giant syringe) BUT once I disconnect the brake hose at the MC I can reverse bleed the fluid right up to the banjo bolt. This says there is something clogged in the mc that won't allow fluid in.

Weird thing is, I can bleed just the mc (no hose) just fine, pressure builds up when I cover the hole with my thumb and even fluid squirts out.

Is the banjo bolt at the mc clean?

The fluid comes through a small hole in the center of the bolt and out through 2 holes in the loop portion of the bolt. If the center hole is plugged it could cause the issue you are having.

[KennyFSU]

Is the banjo bolt at the mc clean?

The fluid comes through a small hole in the center of the bolt and out through 2 holes in the loop portion of the bolt. If the center hole is plugged it could cause the issue you are having.

Herb, yes it banjo is brand new (as well as the hose). Fluid comes out the center hole just fine when reverse bleeding BUT once I connect the hose to the MC I can no longer reverse bleed into the MC. The MC was recently rebuilt by me, I cleaned it out and installed the cups correctly. No idea what else I can try. Thanks.

[Herb]

Is the banjo bolt at the mc clean?

The fluid comes through a small hole in the center of the bolt and out through 2 holes in the loop portion of the bolt. If the center hole is plugged it could cause the issue you are having.

Herb, yes it banjo is brand new (as well as the hose). Fluid comes out the center hole just fine when reverse bleeding BUT once I connect the hose to the MC I can no longer reverse bleed into the MC. The MC was recently rebuilt by me, I cleaned it out and installed the cups correctly. No idea what else I can try. Thanks.

Gotta be something in the MC stopping the fluid. Banjo bolt too long, banjo washers too thin, something inside the MC plugging it up? I have exhausted my ideas.