Ajp Brakes - Caliper Rebuilding And Bleeding

[theflyingferret]

For those as mechanically inept as myself:

My brakes were binding, both the front and the rear. They'd gotten so bad that I could barely push the bike around the garage. The back brake was particularly bad and the disc discoloured with heat after my last ride.

I know a lot of other people's bikes suffer from the same issue. I believe it happens for 3 possible reasons:

1) The Master Cylinder is not functioning properly, Piston or Seal issues. It is not returning after pressure is released and therefore pressure remains o the Pads providing unwanted braking effect.

2) Dot 3 and 4 Brake fluid absorbs water over time. Water in the system corrodes the Master Cylinder's and Caliper's internal surfaces. Usual places for Corrosion in the Calipers include the bottom of the Cylinders beneath the Pistons and on the Grooves of the Seals.

3) The Pads hang up on the Locating Clip, Locating Bolt or on the Caliper casting itself.

After stripping down the Calipers I found that some of the Seals had perished and even been sliced due to contact with the Seal Groove in the Caliper. I ordered 2 rebuild kits...you get the 4 Seals, 4 Pistons and the little sealing O-Ring that sits between the 2 mating halves of the Caliper. As usual I left everything to the last minute (evening before the trial) thinking that the rebuild and bleeding would be an hour's job max...not so, at least not for me.

There are no instructions with the kits for rebuilding and I was too lazy to look online. This resulted in me hastily rebuilding the Calipers and bleeding the brakes only to find I had no brake pressure what-so-ever, yet no sign of air in the brake systems either.

After spending half the weekend covered in brake fluid (and frustratingly missing my trial) I thought I'd write up a half decent lessons-learned (painfully) process for rebuilding and bleeding 4 Piston AJP Calipers that at least worked well for me (in the end). This process comes from the experience of building the Calipers incorrectly and having new Pistons bind on new Seals before stripping and rebuilding again only to have the exact same parts work perfectly.

(I have the 2000 model Rev 3, so it has 4 Piston Calipers at both ends. I believe 2001 and younger models have the 2 Piston Calipers at the back...this process ought to work just as well for those.)

What you'll need:

- A bottle of Dot4 brake fluid.

- A bucket or old washing-up bowl (it gets messy).

- A syringe (mine's ~80ml I think) and 3 feet of transparent hose (needs to be a snug fit on the bleed nipple).

- The kit(s).

- Plumber's (Teflon) Tape.

- Rags.

- Some other stuff based on what you find after stripping your Calipers down...

1) Remove the Caliper(s) from the bike, put rags on the ground beneath the brake lines to catch the drips of brake fluid for when you disconnect the Banjo fittings.

2) Disassemble the Caliper(s) completely. I used compressed air to remove the old Pistons. Use eye protection if you're doing it this way, these things can fly a good distance and at speed. A Vice is handy to hold the Caliper halves. They come out one at a tie, the first is easy, you'll hear the 'pop'. You then need to temporarily re-seat (just barely) this Piston to block the passage it just uncovered in order to get the second Piston out. You can then pull the first one out again by hand.

3) Make sure that the Pad's locating Clip and Bolt are present. These go through the holes in the top of the Pads...one of my Calipers had the Pad locating Bolt missing, which won't have helped with binding in the first place since the Pad and Caliper showed evidence of fouling on each other.

4) Inspect the Locating Clip and Bolt, check the Bolt Shank for burrs...the one Bolt I did have was heavily burred by the Pad being forced into it under braking pressure from the Disc.

5) If burred, either de-burr the Bolt with some scotch bright or fine-grit sandpaper...or get a new Bolt. The Shank needs to be smooth and unobstructed for the Pad to slide along it freely...same with the Clip Also check the surfaces of the holes in the Pads (if you're re-using the exisiting Pads). Mine were pretty rusty, so I used a Dremel tool and fine grinding wheel to clean the up, add a lead chamfer to the Holes and smooth out the surface of the hole walls. You could do the same with a fine circular file. Again, it's just an attempt to make the Pads slide as easily as possible and not hang up.

6) Carefully pry out the old Seals from their Grooves. I used a hooked Scribe but you might get away with a cable tie end or something similar. Be very careful not to mark or score the Groove walls which are used as a Sealing surface against the Seal. You might be planning on reusing Seals if they're still plyable...so don't damage them if you are.

7) Thoroughly clean everything. I put everything in an old washing-up bowl and since there are no Seals to worry about now you can clean the Caliper Castings with brake cleaner. Make sure the Seal Grooves are scrubbed thoroughly as dirt/corrosion build up beneath the Seal is often another contributor to binding. An old toothbrush helps here. Spray brake cleaner through the fluid passages to ensure they are clear.

8) Allow the Caliper Castings to dry out (brake cleaner should evaporate within a few minutes).

9) Carefully seat the new Seals (dry, don't apply any brake fluid yet) into their Grooves. They should go in without much protest.

10) You'll need to sit the new Piston in a shallow bath of fresh Brake Fluid. (I used a Jam jar lid).

11) Pour brake fluid into the Cylinders and make sure the inner diameter faces of the new Seals are well coated.

12) Make sure the new Pistons have good coverage of Brake Fluid on the back face and the outer diameter...and one Piston at a time...

13) Key Step here, fingers only...and where I went very wrong the first time I'm sure:

- Place the Piston into the Cylinder and gently press down as uniformly as possible.

- They should not take a lot of pressure to move. They'll likely bind at points as you're pressing them and it simply means that you're not pushing them down square in the Cylinder.

- If this happens don't force the Piston...gently push on one side of the upper face and change the position you're applying pressure until the Piston moves again. They should glide down smoothly.

- You MUST completely bottom out the Piston in the Caliper Cylinder, I didn't do this the first time around which is why they wouldn't move.

- The Caliper's Cylinders are milled. This makes the bottom of them perfectly (tolerances allowing) perpendicular to the Cylinder Wall.

- Bottoming them out ensures that the Piston is positioned squarely in the Cylinder as we can use the Cylinder bottom to align it. It will then travel freely back and forth.

14) Put the Caliper(s) back together using the new O-Ring (or good old one) from the Kit to seal the 2 halves...make sure that the Locating Clip and Bolt are refitted.

15) Make sure the Bleed Nipple is clean and wrap the threads with Teflon Tape this really helps when it comes to bleeding brakes. Go 4 or 5 times around the threads, I'll leave you to figure out which direction to wrap in...righty-tighty and all that. Refit the Nipple in the Caliper and tighten.

16) Remove the Cap and Rubber Diaphram from the Master Cylinder Resevoir...and making sure that the rag is still in place on the floor below (or use something else to catch the fluid)...start pumping the lever/pedal. Once empty, refill the Master Cylinder Resevoir with fresh brake fluid and pump until empty again. This should purge the system of the old fluid...it's worth doing this, especially if the old fluid is brown in colour which means it has absorbed water..which leads to that internal corrosion of the Calipers/Master and a eventually a ****ty brake.

17) Reassemble the Caliper to the bike and the fit the brake line Banjo to the Caliper.

18) If the lever or pedal is adjusted well-in you'll need to take out that adjustment so that effectively you'd have a very weak brake. I found it was especially important to do this on the front. Don't worry, this is only temporary.

19) Onto the bleeding part then:

- Fit the Hose to the Syringe, push the plunger all the way down and put the end of the Hose in the bottle of Brake Fluid (I did say it get's messy!). Draw the plunger back...you should get a syringe full of fresh Brake Fluid.

20) Now, this part is key too:

- Hold the Syringe/Hose by the tip of the Hose itself so that the Syringe is dangling in the air below.

- You'll see air bubbles trapped in the Syringe and Hose most likely.

- Use the 10mm Spanner (that you're about to use to open and close the Bleed Nipple) to gently tap the Hose where the air bubbles are to get them moving upwards.

- Once the Hose is clear of bubbles, very gently push the plunger (keep the Syringe vertical facing upwards) to force the trapped air from the Syringe into the Hose.

- Again tap with the Spanner until the air comes out at the top.

- Gently push the plunger until the fluid is at the very top of the Hose, about to spill out.

21) Without allowing air to re-enter the Hose fit it to the Bleed Nipple. You may have to apply very slight pressure to the plunger to do this and you'll likely spill some fluid. If you do see a few bubbles in the Hose at this point it's not the end of the world, they'll come out again soon enough.

22) Crack open the Bleed Nipple 1/4 turn. You shouldn't need to open it further than this. The Teflon Tape seals the system from the atmosphere if you only open it 1/4 turn and this is very important as it prevents the introduction of air into the brake system.

23) Gently push the plunger on the Syringe and keep your eye on the Master Cylinder Resevoir...you're looking for fluid to appear as you force it into the Caliper and up the Line. If you haven't taken out the adjustment as described earlier the Piston in the Master Cylinder maybe too far advanced in it's Cylinder and block off the Fluid you are trying to force through to the Resevoir.

24) The front requires more pressure than the rear, but you should start to see Fluid building in the Resevoir...you should see air bubbles coming out also. Easy on teh plunger, unless like me you want your garage ceiling decorating with brake fluid!

25) Once the Master's Resevoir is 3/4 full close the Bleed Nipple. For m 3/4 is effectively full...consider that when you put the Diaphram back in later the fluid level will rise significantly in the resevoir.

26) Pump the lever/pedal repeatedly...you're now advancing those new Pistons in the Caliper onto the Pads, and the Pads to the Disc. It might take a dozen pumps or more but you should start to feel a brake. Keep you're eye on the Resevoir level which is going down and top up if necessary as you're pumping. Don't let the resevoir level drop below 1/4 full or you risk drawing air into the system.

27) There's a good chance that the system is completely free of air and that you have a good brake already. Once you have a brake you just need to ensure that any air that was in the Caliper behind the Pistons is purged.

28) Open the Bleed Nipple 1/4 turn once again. With your resevoir full you now need to just push and pull the fluid back and forth until the rest of the air (if any) comes out. Pull on the Syringe Plunger, keep your eye on the Hose and keep your other eye on the Resevoir for air bubbles.

29) If you have drawn bubbles into the Syringe or Hose close the Bleed Nipple and purge the air from the Syringe/Hose again (Step 20). Then refit it and push the fluid back up through to the Resevoir. Repeat this step until you don't see air bubbles coming out of the system either in the Hose or in the Resevoir. It shouldn't take more than 2-3 repetitions before you have a good brake...you can have a good brake pretty much straight away if you've gotten everything right. If you have no bubbles coming out, you're only moving fluid, which is exactly what you want and as good as it gets.

30) Your brakes are bled. With the Resevoir 3/4 full refit the Diaphram and the Cap. Re-apply your lever/pedal adjustments as you like them.

29) After checking that you have in fact got brakes...you'll find that they're squeaky and rather crap, even though you've got good pressure and resistance. This is because you've gotten brake fluid on the Pads and Disc. It's almost unavoidable...at least it was for me. Wipe it up as best you can with a rag...hose them off even. You'll get the fluid of the disc easy enough but it actually soaks into the Pads.

30) Take the bike out for a ride and make sure you have a bottle of water with you or better yet, ride next to a river or stream deep enough to immerse the Calipers in. You need to get the brakes hot to the point that you can smell them...so drag them for 30secs in 3rd gear at speed or something. Then, with the water you need to quench them. Basically you've burned off the brake fluid in the Pads and then quenched them which changes the molecular make up of the PAd's surface restoring the friction you lost. In turn, this should restore your braking power, feel and get rid of the squeaking...although you'll need to allow enough time to rid the pads/disc of the water you just applied to them first. This might take a couple of goes.

I'm sure the more experienced lads and lasses have more tried and tested methods, but I couldn't find much written up on the subject on here so...there are some decent youtube videos of reverse bleeding MX and Quad Bikes but they don't always tell you why you're doing what you're doing...and they miss some really helpful tips like the Teflon Tape. I couldn't find much out there for actually rebuilding sensitive little Calipers either.

Hopefully someone out there can benefit from my mistakes, errors, frustration and perhaps the rather large paddy I had in my garage on Saturday!

Best of luck...

Edited October 17, 2012 by theflyingferret

[pindie]

A tip for step 13.

Fit a blanking plug into brake line hole.

Fill calliper body with fresh fluid and then put both pistons in only just to be held and create a seal.

Hold calliper vertical so nipple at the top.

Hold a flat steel rule across the partially inserted pistons.

Now open the bleed nipple and very slowly use the steel rule to gently press both pistons home squarely.

Close the nipple and keep the calliper in this position until fitting the pre filled (with fresh fluid) pipe.

All this helps minimise air you have to later bleed out later. Be sure to clean all these filled parts exterior well with brake cleaner before you handle your new pads. Change gloves/wash hands before fitting pads.

[2stroke4stroke]

Good of you to go to the effort of writing this up.

When I made the mistake of pushing the pistons all the way in I had to pull them out again (not an easy job) as it was the olny way to get fluid in behind them to allow the brakes to function.

I use silicon grease in the seal grooves and also spray it on the visible portions of the pistons (obviously without the pads in) as this lubricates the piston/seal interface and also prevents muck getting good adhesion thus helping to prevent corrosion or seizure due to dirt.

[theflyingferret]

Pindie, I see no reason that your method of pushing down the Pistons wouldn't work...it's just a different way of doing it. You'd be surprised how little air you end up trapping in the system to begin following the method I described and what you do end up trapping comes out very easily once you bleed the system. Again, there's no harm in doing it your way either.

2st4st, I'm not sure why you couldn't fill the system with the Pistons bottomed out. The fluid should collect (perhaps with some air) immediately behind the Pistons no matter what their position and then make its way through the rest of Caliper's Gallery, up the Line and and into the Resevoir. Bottomed out Pistons do not block off the rest of the Fluid Gallery. Did you close the Bleed Nipple and pump the lever/pedal enough times to advance the Pistons to the Pads/Disc? I do like the Silicon Spray idea...as long as it doesn't react negatively with the Seals. I only mention this as I followed another poster's recommendation to soak old Seals in WD40 in order to soften them and give them longer life...I did this and found that my Seals had become twice the diameter they were originally! Just a watch out.

Thanks to both of you for the additional tips/methods.