Hi. yesterday i rebuilt my rear brake master cylinder ( NISSIN 14mm bore cylinder ) with a new rebuild kit. then i bled the whole system and brake was working fine. went for a ride and after several kms i felt an excessive play in my brake pedal. came home and checked it. it traveled too far freely before engaging brakes. it was not spongy just moving freely up and down.

so i disassembled the master cylinder again. removed the circlip. well my piston was stuck inside the cylinder. managed to take it out and apparently cylinder surface was fine and only had some dark color markings at the end of the cylinder left by piston.

this is the type of rebuild kit i had



this part of the piston had touched the end of the cylinder




this is the markings left on the cylinder wall



this is the first time i encountered with this type of issue. i measured the piston OD and it was 13.8mm. so what could have made this piston stuck inside the cylinder. could it be a faulty seal which is fitted on the piston? i bought a new rebuild kit but haven't assembled it yet. can someone help me please. thank you

 

Follow this guide: https://www.partzilla.com/blog/honda...linder-rebuild

From scratch mark on side of cylinder, something is not concentric and even all way around.

1. with everything removed, shine light from small end of cylinder and look down
bore, is it perfectly straight? Is where spring seats perfectly circular and flat?

2. measure piston along edges length-wise to verify it's straight. Measure OD at various rotations to verify it's actually round. Wear marks on cylinder shows something's crooked. Slide piston into cylinder and verify you can move it entire length of cylinder without binding. Rotate it about 30-degrees and try again. Then another 30-degrees, etc.


3. test spring seat. Push spring large end 1st all way to end. Compress it slightly and release gently so it doesn't jump off seat. Is smaller end centred perfectly in bore?

4. step#2 guide has you snapping black seal onto tapered end of spring. Problem is when you insert into cylinder, larger diameter of seal aways pops it off end of spring and they may not centre properly. I find little dab of contact cement helps hold seal onto end of spring.

Lube cylinder with brake fluid and push spring in until seal touches. Now, here's tricky part. Using small screwdrivers, toothpicks, etc. While gently pushing down centre, work your way around edge of seal and press it inwards gently so goes into bore. Very gentle so seal remains centred and flat. When you have entire edge seated into bore, then shove it down

5. finish rest according to guide

 

@dannoxyz thank you so much these tips and tutorial link are quite handy. will check the cylinder and install rebuild kit according to your guidance. will pay more attention this time. will update soon.

 

I think in step 6 of that guide they forgot to add "replace the circlip"?

 

hi. i rebuilt with a new kit and bled the system. went for a short ride and everything was working fine. thank you