Trying to understand rear brake anatomy

[chilly_75]

I have been trying to bleed my rear brake for a few days now, and frustration is mounting. I have cleaned all the lines and reservoir, and rebuilt the master cylinder. It appears to be sealing and functioning appropriately. I find no fluid leaks in the system.
Once I suck fluid through the line using a hose connected to the rear bleeder valve, and close it back up, there is still zero resistance in the pedal.
I took the reservoir cap off and watched as I pumped the pedal. The fluid level rises and falls with each pump, as if fluid is not being pushed down the line, but rather back into the reservoir.
I compared to my front reservoir, and noticed something appears to be missing. Confirmed with manual that I do not have the clamp plate in the bottom of the reservoir on the rear. (See attached pic to show my two reservoirs and what the manual shows for the rear system).
Is there a check valve or something that prevents the master cylinder from returning fluid to the reservoir, thereby forcing fluid down the line?

<UGH! The forum is not letting me upload the pic. I just keep getting an error!>

 

[Gorminrider]

ASSUMING you have the piston seals and Master Cylinder rebuilt correctly, It sounds like you still have an airbubble in the line. It can be quite hard to "reprime" the system. Yes, There are special patent tools to buy if you want to make it really easy... I don't have one - the expenditure of a little time and playing with funk seems to do as well....

There is no "checkvalve" in the braking system. If there were, your brakes would lockup everytime you used them. So you have to "emulate" a checkvalve when you bleed brakes. (or buy a doofunny)

Air can sit in the the line or the reservoir moving backnforth,backnforth. It's compressible too, so it acts like a spring...(as in "spongy" brakes which indicate air in the system) Air rises in the line so I usually imagine it to be up at the MasterCylinder and that's a long way from the bleeder valve..Trying to suck it encapsulated by fluid all the way down to Caliper is a very long haul ...several homemade things can help...I have used big plastic "pet syringes" to suck these out at the master cylinder (not down at the caliper, but you can try) ...ie suck up slowly at the relief hole, let the air float up in the syringe and then push the fluid back into the reservoir )....Another way without the syringe is to get things working before installing...ie getting master and line full of fluid and working before attaching to a caliper (also as full as possible). This really does help a lot.

as in: undo the lowest end of the brakeline , get it above the MC so the bubble IS escaping as you "pump", then put it quickly back on to brake caliper (prefilled as much as possible) and commence bleeding before any bubble at that lower connection has a chance to return back up to the Master.
Meanwhile the Brake caliper's bleeder is positioned so as to allow any air in the caliper piston to rise up to it so air is usually easy to get out there.

You can BUY a bleeder valve with a checkvalve installed in it and tire valves can work too, inserted in the hose you are using to bleed with...these saves a lot of opening and closing in unison with moving the Master's piston.plus have the advantage per airbubbles that it's faster, and might be faster than that air bubble trying to rise.

 

[chilly_75]

Thanks for all the detail, gorminrider. I will:
A) Keep working on it
B) Try these tips
C) Take deep breaths and be patient

 

[hillsy]

If you are using a syringe to suck the fluid out, try pushing fluid back through the line and out the reservoir.

 

[Steve]

Agreed, there is no checkvalve.

Sounds like the rubber diaphragm on the piston might be installed incorrectly or there is a HUGE air bubble stuck somewhere.

My procedure for priming a dry system:
1. Remove hose from top of master cylinder.
2. Slowly push down on pedal, watch for fluid squirting out top of cylinder.
3. With pedal still DOWN, put finger over hole, release pedal.
4. Repeat, possibly with finger lightly over the hole, acting as a checkvalve.
5. When fluid flow is assured, put banjo fitting back on and tighten.
6. Open bleeder valve on caliper, find the spot where you can open and close it easily.
7. With bleeder open, push on pedal.
8. Close bleeder, release pedal.
9. Repeat as necessary until fluid comes out steadily from bleeder.
10. Repeat with other bleeder.
11. Check overall pedal feel.
12. When the pedal feels good, run bike on centerstand, check brake operation.
13. Go for a test ride.

.

 

[mnferwerda]

My son and I have bled brakes with much frustration trying to get it primed. We finally bought something like this:

https://www.amazon.com/Mityvac-MV80...&qid=1487892232&sr=1-7&keywords=brake+bleeder

So much easier and quicker.

 

[mnferwerda]

I also use it to check and calibrate my carb snych gauges

 

[jsandidge]

I also use it to check and calibrate my carb snych gauges

That's a great idea!

 

[hillsy]

A syringe and a bit of tube will only cost you a buck or two.

Just saying.

 

[GSXR7ED]

Front brake, rear brake...Same, same.

I did MC rebuild on my single front disc brake in 2013 (I have rear brake shoes) and I soaked the parts in solvent over 48hrs and it cleaned up those tiny holes in the MC so I could bleed with no air bubbles--I also put a new braided line on. It's painstakingly difficult to get rid of the air pocket but keep pumping the bleeder until you reach your goal. Good luck!


Ed

 

[bobgroger]

Also, the threads around the bleeder valve are quite leaky. Put some wheel bearing grease around the threads, and use a vacuum pump. I think you can borrow one at Autozone

 

[Gorminrider]

Thanks for all the detail, gorminrider. I will:
A) Keep working on it
B) Try these tips
C) Take deep breaths and be patient

Righto..... Reading all these other posts, I think it's about evolving a technique that works for you. ...just keep that airbubble in mind.

If I had to do brake dissassembles often, to pump fluid up from the Caliper's bleeder right up and through the Master would seem ideal .

A cool storebought thing as shown would be nice. but a big pet syringe( for dosing cows etc) is awfull handy and cheap.... HOWEVER the rubber seal in them tends to swell up tight after being used once . If you use them, dissassemble and clean them up after use. Store them plungers out or they can glue themselves together! (wrong rubber for brake fluid, I guess)

 

[bwringer]

Steve's process touched on a version of bench bleeding adapted to rear brakes. Perhaps y'all would benefit from a little more info...

Let's cover front brakes first, since that's a little easier.

When you have a dry master cylinder, it's full of air, not brake fluid. You have to get all or most of that air out before it will be able to pump any brake fluid at all down to the calipers. You need to do this before hooking up the brake line. Here's how to bench bleed the front:

1) Start with the master cylinder clamped to the bars or in a vise.

2) Remove the brake lever, if installed. Remove the brake line(s) if installed.

3) Put some brake fluid in the reservoir.

4) Cover the outlet (where the brake lines were attached) with a finger. You can also place a length of tubing into the outlet, and curve it up and back into the reservoir.

5) Using a strong dowel (or being very careful with a large Phillips screwdriver), slowly push the piston all the way in and let it return several times. You'll see a lot of bubbles come out, and you may need to add fluid. Keep going until all the bubbles are burped out. When you're almost done, you'll start seeing a strong "jet" of brake fluid swuirt up from the small relief hole at the bottom of the cylinder when you push. (Careful it doesn't make a mess.)

You'll need to use your finger over the outlet as a one-way valve, letting fluid escape while doing your best to keep air from getting back in (you don't have to be perfect.) This is messy, so if you can find some tubing that you can jam in there it's a little more convenient.

Once you don't have any more bubbles burping up from the innards of the master cylinder, then you can install the brake line and lever and bleed normally.


For the rear master cylinder, you have to do much the same, except that the cylinder is oriented up and down instead of horizontally. The main points to remember are:
1) disconnect the pedal or any other parts in the way and use a blunt tool to FULLY compress the piston in the bore. (You can't compress it fully with the lever.)
2) Use your finger as a one-way valve over the outlet, or use a piece of tubing filled with brake fluid.