1980 Rear Brake problem.

[mnmailman]

My rear brake seized a few days back. Took apart, cleaned up the master cylinder and caliper and rebuilt the caliper with a PartsNMore kit. M/c was ok.
I've messed with brakes my fair share but this is a new one for me. Got everything back together, have normal fluid pressure/volume at the banjo, fluid going into caliper piston bore...........but the piston won't move. I can easily get it out/move it via compressed air.
However, the pedal will not firm up which is strange too since the piston won't move. Only thing I can think is air in the line, but as said it's been bleed to the caliper banjo.
Ideas ?

[WingAdmin]

Check the return port in the master cylinder. More often than not, a seized caliper piston is actually a blocked return port, which stops the piston from retracting, because there's nowhere for the brake fluid to go. If you rebuilt the caliper, and now you can't get it to retract, it's likely the return port in the master cylinder is the culprit. A very thin wire (i.e. guitar string works well, or clip a single bristle from a wire brush and hold it in needlenose pliers) can usually be used to clear it.

[mnmailman]

Check the return port in the master cylinder. More often than not, a seized caliper piston is actually a blocked return port, which stops the piston from retracting, because there's nowhere for the brake fluid to go. If you rebuilt the caliper, and now you can't get it to retract, it's likely the return port in the master cylinder is the culprit. A very thin wire (i.e. guitar string works well, or clip a single bristle from a wire brush and hold it in needlenose pliers) can usually be used to clear it.

I cleaned it out well during the rebuild and the problem now is it won't even advance; I can't get pressure built up even though fluid is reaching the caliper.
I'm going to take it all apart (again) tomorrow and re-check everything.

[WingAdmin]

Check the return port in the master cylinder. More often than not, a seized caliper piston is actually a blocked return port, which stops the piston from retracting, because there's nowhere for the brake fluid to go. If you rebuilt the caliper, and now you can't get it to retract, it's likely the return port in the master cylinder is the culprit. A very thin wire (i.e. guitar string works well, or clip a single bristle from a wire brush and hold it in needlenose pliers) can usually be used to clear it.

I cleaned it out well during the rebuild and the problem now is it won't even advance; I can't get pressure built up even though fluid is reaching the caliper.
I'm going to take it all apart (again) tomorrow and re-check everything.

When you put the piston back into the caliper, did it press in easily with your fingers?

[mnmailman]

Check the return port in the master cylinder. More often than not, a seized caliper piston is actually a blocked return port, which stops the piston from retracting, because there's nowhere for the brake fluid to go. If you rebuilt the caliper, and now you can't get it to retract, it's likely the return port in the master cylinder is the culprit. A very thin wire (i.e. guitar string works well, or clip a single bristle from a wire brush and hold it in needlenose pliers) can usually be used to clear it.

I cleaned it out well during the rebuild and the problem now is it won't even advance; I can't get pressure built up even though fluid is reaching the caliper.
I'm going to take it all apart (again) tomorrow and re-check everything.

When you put the piston back into the caliper, did it press in easily with your fingers?

Yes, slight resistance via the seal but not much. Comes out easy via a small amount of air too. I did clean out the seal groove and it's spotless, new seal is flush with the bore wall, etc. but I plan on taking another look at that just to be sure it's not binding the piston somehow.

[mnmailman]

Slight update. Finally got the caliper piston to move out as if applying the brakes. Now the problem is it still won't firm up. When it meets resistance ala the brake pads/rotor the piston shimmies very slightly back and forth vs. continuing to move out of the bore and firming up the pedal.
I have solid, no air bubbles fluid coming out of the bleeder.
Each part of the entire system is working as should; banjo bolts, line itself, m/c return port, etc.
HOWEVER, in re-inspecting the m/c I notice the small cup that fits on the piston end where the push rod ball end contacts it (not the spring end) is missing. It must have never been there as I positively did not lose it (other parts were missing so no surprise).
Could this missing cap be the cause of the piston shimmy ? Not sure how it could be as I figure it's just to prevent wear from the push rod ball end on the piston receiving cavity but maybe I'm mistaken.

[WingAdmin]

Slight update. Finally got the caliper piston to move out as if applying the brakes. Now the problem is it still won't firm up. When it meets resistance ala the brake pads/rotor the piston shimmies very slightly back and forth vs. continuing to move out of the bore and firming up the pedal.
I have solid, no air bubbles fluid coming out of the bleeder.
Each part of the entire system is working as should; banjo bolts, line itself, m/c return port, etc.
HOWEVER, in re-inspecting the m/c I notice the small cup that fits on the piston end where the push rod ball end contacts it (not the spring end) is missing. It must have never been there as I positively did not lose it (other parts were missing so no surprise).
Could this missing cap be the cause of the piston shimmy ? Not sure how it could be as I figure it's just to prevent wear from the push rod ball end on the piston receiving cavity but maybe I'm mistaken.

I'm not sure which cup you're talking about.

There is a metal piece that fits over the top of the spring:



Then there is a seal that fits over top of that:



Lastly, there is this concave "cup" on the end of the piston in front of the seal - however, this cup is PART of the piston, so if it's missing, it means your piston is actually broken:

[mnmailman]

I have that part on my piston. I think I'm mistaken about a missing part though; I've been looking at the factory service manual and rebuild kits schematics and they have this part separate, OFF the piston, not on it.
I didn't notice that until now and seeing it separate made me think if was another part itself.
I.e., with this PartsNMore kit example for the rear m/c from a 1980.https://www.partsnmore.com/parts/honda/ ... ng]=custom

[mnmailman]

Maybe I'll just leave the pedal applied overnight and see what happens.

[mnmailman]

On another note, someone in the past apparently didn't have snap ring pliers. Few nicks and chips on the lip of the m/c where the snap ring holding the push rods goes, looks like someone used the m/c lip as a screwdriver leverage point to get to the snap ring.

p.s. thanks for the m/c photos. Helped clear up the "missing part"

[mnmailman]

Just to be clear; can fluid ONLY be returned to the resevoir via the small hole or can it also go via the larger hole (next to the small hole) ?
By pushing on the caliper piston I can get fluid to go back into the resevoir so it would seem the return hole is clear, but just want to make sure it is going via the correct path. I did clean out the hole.
Also, to my knowledge a plugged small m/c fluid return hole would only result in locked brakes as the fluid couldn't get back to the resevoir so the brakes never release.
However, would this hole being plugged have any affect on brake bleeding ? (other than locked brakes). I.e., would it being plugged not allow air to escape the system ?
I don't see how this could be the case but want to make sure.

[WingAdmin]

Just to be clear; can fluid ONLY be returned to the resevoir via the small hole or can it also go via the larger hole (next to the small hole) ?
By pushing on the caliper piston I can get fluid to go back into the resevoir so it would seem the return hole is clear, but just want to make sure it is going via the correct path. I did clean out the hole.
Also, to my knowledge a plugged small m/c fluid return hole would only result in locked brakes as the fluid couldn't get back to the resevoir so the brakes never release.
However, would this hole being plugged have any affect on brake bleeding ? (other than locked brakes). I.e., would it being plugged not allow air to escape the system ?
I don't see how this could be the case but want to make sure.

The small hole is used for return only - if it is plugged, you can still pump pressure to the brakes, as well as bleed them.

[mnmailman]

Just to be clear; can fluid ONLY be returned to the resevoir via the small hole or can it also go via the larger hole (next to the small hole) ?
By pushing on the caliper piston I can get fluid to go back into the resevoir so it would seem the return hole is clear, but just want to make sure it is going via the correct path. I did clean out the hole.
Also, to my knowledge a plugged small m/c fluid return hole would only result in locked brakes as the fluid couldn't get back to the resevoir so the brakes never release.
However, would this hole being plugged have any affect on brake bleeding ? (other than locked brakes). I.e., would it being plugged not allow air to escape the system ?
I don't see how this could be the case but want to make sure.

The small hole is used for return only - if it is plugged, you can still pump pressure to the brakes, as well as bleed them.

Thanks again; you've been a huge help.

[WingAdmin]

Did this end up fixing your problem?

[mnmailman]

Did this end up fixing your problem?

I just cannot get the rear pedal to firm up. It does apply/release the brakes but with to much pedal travel to do so. I ordered/installed a Hel stainless steel line too. Btw, strange experience with them which is unusual; didn't send new banjo bolts/crush washers with the line.
Anyway, wife and I have bled the line for hours and hours. Good, bubble free fluid coming out of the bleed nipple but the pedal just won't firm up at the same point it used to.
I rebuilt the caliper but not the m/c; it's parts measured to spec and seems to be working.
Taking a break from it the rest of the day today.

[WingAdmin]

Did this end up fixing your problem?

I just cannot get the rear pedal to firm up. It does apply/release the brakes but with to much pedal travel to do so. I ordered/installed a Hel stainless steel line too. Btw, strange experience with them which is unusual; didn't send new banjo bolts/crush washers with the line.
Anyway, wife and I have bled the line for hours and hours. Good, bubble free fluid coming out of the bleed nipple but the pedal just won't firm up at the same point it used to.
I rebuilt the caliper but not the m/c; it's parts measured to spec and seems to be working.
Taking a break from it the rest of the day today.

Try putting a bungee cord or something heavy on the brake pedal. Pump it up several times until you get some pressure, then put the weight/bungee on to compress the pedal down as hard as you can get it. Let it sit like that overnight and see if it helps at all.

If not, then it's likely time to rebuild the master cylinder, the rubber seals might be tired.

[mnmailman]

Did this end up fixing your problem?

I just cannot get the rear pedal to firm up. It does apply/release the brakes but with to much pedal travel to do so. I ordered/installed a Hel stainless steel line too. Btw, strange experience with them which is unusual; didn't send new banjo bolts/crush washers with the line.
Anyway, wife and I have bled the line for hours and hours. Good, bubble free fluid coming out of the bleed nipple but the pedal just won't firm up at the same point it used to.
I rebuilt the caliper but not the m/c; it's parts measured to spec and seems to be working.
Taking a break from it the rest of the day today.

Try putting a bungee cord or something heavy on the brake pedal. Pump it up several times until you get some pressure, then put the weight/bungee on to compress the pedal down as hard as you can get it. Let it sit like that overnight and see if it helps at all.

If not, then it's likely time to rebuild the master cylinder, the rubber seals might be tired.

I was thinking about those seals too, sounds like a good idea. Can't hurt. I'll get a m/c rebuild kit in a few and give it a try, thanks.

[globalfx]

I got a recall on the rear brakes!

[Rambozo]

I got a recall on the rear brakes!

Good for you. However, the OP's bike is 27 years older than yours.

[Quintin]

I can't remember, does the rear brake link with one front disc in 80. If so bleed the front caliper.

[WingAdmin]

I can't remember, does the rear brake link with one front disc in 80. If so bleed the front caliper.

No it does not, 1983 was the first year for this feature.

[mnmailman]

I've ordered an m/c rebuild kit so when I get that I'll have a rebuilt m/c, new ss brake line (already installed), and rebuilt caliper (already installed). WingAdmin, if the m/c seals are bad, am I correct in this scenario:

- a small amount of fluid leaks past the seals while still staying in the m/c bore vs being contained in the smaller bore area sealed by the seals. Thus there would be less pressure available.
I.e., larger volume area the fluid is occupying in the bore equals less fluid pressure.

[mnmailman]

Rebuilt the m/c and the brake is better than new. Rebuilt caliper, new ss line, rebuilt m/c. Celebrated by going on a quick 700 mile trip.
Now to do the same on the front. I've already flushed the old fluid out and replaced with new, rebuilt the calipers, and the hoses are only a few years old but figured I might as well install ss lines and rebuild the m/c too.
That way both sets of brakes are "new" from the m/c s to the calipers.

[arnoldsq]

I just did my rear break complete rebuild. This was a lifesaver doing it by myself. https://www.amazon.com/dp/B08CZ4MKW4/?tag=goldwingdocs-20 ... NrPXRydWU=

[mnmailman]

I just did my rear break complete rebuild. This was a lifesaver doing it by myself. https://www.amazon.com/dp/B08CZ4MKW4/?tag=goldwingdocs-20 ... NrPXRydWU=

Thanks ! Fortunately, my wife doesn't mind being a pedal pusher when brake bleeding.