Rear brake problem - RD400F

[SoCal250]

I got the caliper all cleaned up with the picks and then ran a small piece of scotchbrite through the groove. Cleaned it up well.
It's reassembled and back on the bike. Next to check the master and then bleed.

EDIT --  This thread was split from Turning Wrenches »Rear caliper seized. What now? since this issue has grown into a topic of its own and does not involve a stuck caliper

[Yamaha 179]

Let us know how the master cylinder comes out.  The bleeding should be a piece of cake.
Lyn

[SoCal250]

No dice.
Caliper is rebuilt and recently painted. Completely disassmbled, cleaned, painted, and reassembled the rear master. Reassembled the system with new crush washers. It looks great but doesn't work at all. It won't build any pressure with the master and I can't pull any fluid from the caliper even with a Mity-Vac. This rear brake has been the biggest exercise in frustration over the past few years.

[Evans Ward]

Try reverse bleeding the rear caliper.

[Diablo007]

@SoCal350

Might sound dumb, but did you clear out the bleed screw?  I've run across some that were plugged solid in the past and had to use safety wire to clear out the clog.  One of my mentors also had me start bleeding from the banjo bolt on the master to get things started in the past.  In other words squeeze the lever (or press the brake pedal) and crack the banjo bolt loose to let air out.

[SoCal250]

Try reverse bleeding the rear caliper.

That may be next. I'll need to replace the speed bleeder with a standard one and purchase a reverse bleed kit.

Might sound dumb, but did you clear out the bleed screw? 

The bleed screw is a brand new Goodridge speed bleeder. The only component of the entire system that has not been touched and/or replaced with new is the aftermarket braided SS hose.

Attached photo shows the order the parts were assembled in the master.

[pidjones]

Yes, pre-bleeding the master at its banjo is probably your missing link. Without that, they won't draw fluid from the reservoir to pump into the system. In other words, "prime the pump". Vacuum bleeding can only put a maximum of 1 atmosphere pressure on the fluid to move it past the spring-loaded piston. Get fluid in there, and you have many atmosphere's pressure available to move the fluid.

[SoCal250]

Spent several more hours on it. No love. 
-Left the pedal depressed overnight.
-Removed the master banjo and I can see the fluid rise to the top of the bore and barely spill out when the pedal is pressed down. When the pedal is released the level drops back down inside.
-Cracked the caliper banjo and a tiny bit of fluid seeps out.
-Removed the bleeder screw and a little fluid squirted out when the pedal was depressed.
-Tried pulling fluid through again with the Mighty-Vac but only got a tiny bit.
-Swapped speed bleeder out and installed regular bleeder with teflon tape on threads. No change.

What's been done so far:
New master rebuild kit
New caliper seal kit
New reservoir hose
New copper washer on reservoir fitting
New copper banjo crush washers, x4
New Goodridge speed bleeder (also tried clean stock type bleeder with teflon tape on threads)
Master disassembled and cleaned twice -- ultrasonic, brake cleaner, air, small pick. Bore clean and smooth, both holes confirmed clean and clear.
Caliper disassembled and cleaned twice -- ultrasonic, brake cleaner, dental pick. Bore confirmed clean and smooth, All passages clean. Piston is clean and smooth.


-Reverse bleeder kit will be here Tues 10/13. I may not wait that long.
-I have an RD400G rear master here which is the same part, which I could also try but it will need to be cleaned up and rebuilt first.
-I also have a NOS Ducati Brembo master that I may try but I need to make a mounting bracket first, which will be difficult since I don't have much in the way of equipment for fabbing. Reservoir fitting is also a different diameter (smaller). I purchased it a while back with the intention of upgrading one of my RD400 rears at some point.

[85RZwade]

I've had good results pushing fluid into the bleeder fitting on the caliper with a large syringe. You have to watch the fluid level in the reservoir so that it doesn't overflow. Syringe is cheap at a farm supply store.

[Evans Ward]

I'm confident reverse bleeding will solve your issue. Like you, I tried many methods on my rear stock RZ brake caliper before trying reverse bleeding. I bought a nice kit off Amazon that I've also used successfully for an automotive application so I'm sold on reverse bleeding. As mentioned, you don't have to spend the big bucks on a kit and can use a large animal syringe or the like.

[1976RD400C]

I've never been able to bleed a rear brake on a RD without doing the reverse bleed, push fluid in the caliper and keep going until you see it come up to the reservoir.

[SoCal250]

Starting to get the idea there is a black cloud hanging over this brake system. The USPS package with my reverse bleeder kit, which I have been eagerly awaiting, was to be delivered today. However, I watched the mailman drive up (6-8 hrs later than normal) and put only regular mail in my mailbox, no package, and drive off. Went out and looked and sure enough no package. Yet, the tracking was marked as "Delivered In/At Mailbox" with the same time stamp as when the postal moron was at my mailbox.

This isn't something new, mail delivery and accuracy has been a big shortcoming for the past 3-5 years. This is the 3rd time this has happened this year. Only 1 of the previous 2 missing packages was ever found. So I'm still without a brake bleed kit unless it miraculously shows up somehow.
F*ck the USPS!

[pidjones]

Our mail came four hours later than normal due to COVID + Columbus Day + weekend combination. Our carrier has delivered to wrong address, then gone back and corrected. Not an easy job, but would be appreciated if more care was taken. And if the package is really needed, seems you can count on late/incorrect/lost. But the advertisements will be there!

[SoCal250]

Ordered a 2nd reverse bleed kit. Only 2 days for delivery and this time I saw the guy scan it at my mailbox. (Shipment #1 is still MIA but I filed a missing shipment claim with USPS.)

So...the 'magic' reverse brake bleeder works great. But it didn't solve my problem. I pushed fluid through 3 times -- 3 full reservoirs of fluid from the caliper. Still no pedal resistance and the caliper piston doesn't move. Didn't notice any air bubbles coming up in the reservoir cup. And I have no leaks, which I guess is good, considering. I even unbolted the caliper and dropped it down to ground level and put my hand on the piston when pressing the pedal and didn't feel anything.

Well, now for plan B...err I mean plan J. Plan B was a long time ago.

[pidjones]

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.