Rear brake problem - RD400F

[SoCal250]

Sounds like (sorry) the MC is assembled wrong. Look at some photos of disassemblies. I had one with the rear cup in the wrong position (from the vendor), and another where the cup slipped into the wrong spot because I didn't use brake fluid to lubricate it all on assembly. Most MCs have two rubber cups - one on the outlet spring and one on the piston rod. Both open towards the outlet. On the RDs, there is also a thin disk that works as a check valve between the front cup and the end of the piston. Make sure the disk is present, flat, and the holes in that end of the piston are clear. And that the second cup is behind the knob on the piston rod, not slipped forward past it.

Thanks for the continued input and help

See the photo above from Oct 7 for the assembly order used.

The holes in the piston head should be clear. It was run through the ultrasonic several times during this long ordeal and during the last assembly I ran a fine copper wire strand through the holes to ensure they were open.

[Evans Ward]

To be clear- what bike is this on? I've seen the rubber cup under the spring invert incorrectly upon assembly. I use my pinky finger to seat it so that doesn't happen.

[SoCal250]

To be clear- what bike is this on?

RD400F rear  (Note: I just split this into a new thread of its own since it was originally part of another topic on brakes)


Latest non-progess report...
Today I removed the master again and confirmed the internal parts were in the correct order. Everything looked OK. However I decided to rebuild it again with a new OEM piston seal and rubber cap. Parts were installed in the same order as shown in the Economy Cycle reference image. (see attached photo)  During assembly I verified the rubber cap was inserted with the correct orientation and did not flip over. Both the seal and the rubber cap have the "open" end of the seal flange facing the brake line as shown in the photo.

Also removed the caliper and confirmed the piston is free and can be pushed in by hand. I put a small piece of wood in it and put 25psi compressed air into the caliper to confirm it would move out, which it did.

Reassembled everything, reverse bled twice, then traditional bled sveeral times but still no dice. At least there is a little firmness in the pedal now and I can feel/see the caliper piston moving slightly. There is still not enough pressure to firmly clamp the pads to the rotor so they rattle if the caliper is rocked back and forth.

Going to let it sit overnight and tackle it again tomorrow. 

[Diablo007]

Oddly enough I had a similar problem this past weekend with a rebuilt master on an RD350.  I ended up removing the hard line from the caliper, put my finger over it and squeezed the lever, released my finger, put my finger back over it, and repeated until pressure built up in the line enough to squirt fluid when I removed my finger.  At that point I reattached the hard line and was able to bleed the brake.  It seems at this point it may be worth a try.  You could also remove the master and attach it to a piece of handle bar and so the same thing with the master only, finger over the banjo bolt hole.  In hindsight that probably would be better as you won't risk brake fluid on your newly painted caliper.  I wanted to make sure the brake line wasn't the problem and with an RD350 removing all the fittings to test would have been a pain.  Probably not so much for the rear brake system on your RD400.

[m in sc]

^ i wind up doing this more often than not, its good advice.

[pidjones]

^ I've had to do this on new Chinese MCs as well as rebuilt Honda MCs. I call it priming the MC.

[SoCal250]

Disconnected the hose at the caliper. Squeezed in between my fingers and pressed the pedal a few times. Couldn't hold back the pressure with my fingers so I succeeded in spraying/misting brake fluid in my work area and on the rear wheel. Bolted it back up tried bleeding a few times. Still not much resistance on the pedal and the caliper piston barely moves (but does not slide out of the bore and firmly contact the pad) when pressing the pedal down.

[Diablo007]

At this point it the problem seems to be the brake caliper itself.  To me it sounds like there's a leak letting the air in the caliper out and then back in as you haven't mentioned brake fluid leaking from any caliper seals.  I'd take the caliper apart and inspect the seals and sealing surfaces.

[1976RD400C]

A few more thoughts:  Is the pushrod adjusted to have that little bit of play that makes sure the piston is coming back all the way? When you push the petal down and then open the bleeder, close the bleeder and let the petal back up slowly and wait a little while before touching the petal again. Before you do go to push it again gently tap the petal several times so the pushrod is just barely moving the piston. Then continue with a full push of the petal.

[SoCal250]

At this point it the problem seems to be the brake caliper itself.  To me it sounds like there's a leak letting the air in the caliper out and then back in as you haven't mentioned brake fluid leaking from any caliper seals.  I'd take the caliper apart and inspect the seals and sealing surfaces.

The caliper has been apart a few times. Earlier this month I disassembled, cleaned and inspected it again. Made sure the seal groove was clean, bore and piston are smooth and clean with no pitting or corrosion, lubed the new seal with red rubber grease, and put it back on the bike. I plan to remove the caliper again to disassemble, inspect, and reassemble. I may even put another seal in it just because.

A few more thoughts:  Is the pushrod adjusted to have that little bit of play that makes sure the piston is coming back all the way? When you push the petal down and then open the bleeder, close the bleeder and let the petal back up slowly and wait a little while before touching the petal again. Before you do go to push it again gently tap the petal several times so the pushrod is just barely moving the piston. Then continue with a full push of the petal.

There is free play in the pushrod. I tried slow pedal depresses with a pause between each depress. In fact, I've tried just about every method I could think of. I also waited 10-15 min between each bleed attempt. I'll give it another go sometime this week. But it's seeming like Groundhog Day.

[Diablo007]

Might be time to test with a known good caliper if you have one, or a different known good master.

[jradnich]

These are some longshots but things I have seen over the years:
The bleeder isn't compatable with the caliper. It fits and is open but fluid won't flow.
Same with the banjo bolt, the holes are free but won't line up with the opening in the banjo fitting.
The brake line is weak internally and when bent it collapses and blocks flow.

Try taking the bleed screw out and letting it gravity bleed, if you don't get fluid try it at the banjo fitting.

[Diablo007]

At this point we know the master is good.  Sorry, forgot you tested it.  If you're 100% that the caliper rebuild is good these are the things that come to mind.

Being an IT guy I've learned (and relearned) that we sometimes forget the obvious:

Is this the original system that was on the motorcycle?

Was it working before the rebuild?

If so, what was the reason for the rebuild?

Was the hardline to the caliper replaced?

If so does it fit properly?  (In a thread I started regarding brake calipers it was mentioned the RD350 front brake and RD400 rear brake look similar but use different hard lines.  Something I had noticed when going through milk crate full of miscellaneous masters and brakes in the shop.)

As we know the master builds pressure if we assume fluid/air is moving in the line we can test for leaks in the system.  Since you haven't noticed any brake fluid leaks, test the same way you would during a leak down test.  Spray soapy water on the fitting, then pump the brake lever and look for bubbles.  I'd start with the easy stuff, the hard line junction and the bleeder bolt.  The caliper would be a bit of a pain.

Hoping this will help!  If not, I'm at a loss.

[SoCal250]

Is this the original system that was on the motorcycle?

Was it working before the rebuild?

If so, what was the reason for the rebuild?

Was the hardline to the caliper replaced?

If so does it fit properly?  (In a thread I started regarding brake calipers it was mentioned the RD350 front brake and RD400 rear brake look similar but use different hard lines.  Something I had noticed when going through milk crate full of miscellaneous masters and brakes in the shop.)

Yes, caliper and master are stock originals.

Yes, it was working before the rebuild and it has worked ever since I bought the bike in about 2006. The entire system has been on the bike since I bought it. The only replaced items are: new OEM Yamaha master rebuild kit (2 different ones), new K&L caliper seal kit (2 different ones). new copper crush washers, new banjo bolt on master.

It was taken apart for rebuild because the master seal was leaking and causing crusty crystals to form around the bottom of the housing.

There is no hardline. This is a rear RD400F Daytona. The original hose has been replaced with a braided SS line. (refer to photos I posted on Oct 7 above)

The master is unique to the Daytona but uses the same rebuild kit as the '76-'78 1A1 models. The caliper is unique to the Daytona (1-year-only unit, and front and rear are not interchangeable). The bleed screw currently installed is the original and is the correct M8 x 1.25. Banjo bolts are the correct M10 x 1.25.

There are no leaks anywhere in the system currently.  All banjo washers have been replaced with new copper. The master is clean and dry and not showing signs of leaking. The caliper also shows no signs of leaking.

[pidjones]

I had a bike that a PO had replaced a banjo bolt. But, there wasn't a hole in it!