brake question from a newbie..., my search didn't answer my question.. Options

[red914]

first i will try to describe the problem in a meaningful way. i have newer calipers in the rear (from PO). the right rear has developed a drag problem. it does not seem to release, and hence, drags, heats up, etc. I have tried to decipher the haynes manual, but to no good result. does the brake proportioning valve have anything to do with this? how does one adjust the venting? i messed with that a bit, but the allen screw and lock nut allwo brake fluid to seep out of from arond them when screwed in, and only seem to tighten up when run out. none of which, by the way, effects the drag at all.

i realize this is a rambling question, but i am in over my head at this point. any knowledge out there?

[Joe Ricard]

Step 1: set the venting gap as called out in the manual. Yes you may get a little fluid when you loosen the lock nut. The allen screw should go in the tighten (righty tighty) direction to pull the pad away from the rotor. Do this on both inner and outer pads.

Damn this is tough to type after laying 200 brick this evening. and now I gots a beer buzz going WHOO HOOO!!!!! (IMG:style_emoticons/default/beer.gif) (IMG:style_emoticons/default/beer.gif) (IMG:style_emoticons/default/beer.gif) (IMG:style_emoticons/default/beer.gif) (IMG:style_emoticons/default/beer.gif)

[rhodyguy]

disconnect the parking break cables, and take out the springs in betwwen the discs first. you may have to adjust the cables at the firewall. the triangular piece that connects the cables to the handle should be somewhat parallel to the lower interior firewall. print out the pelican tech article.

kevin

[red914]

ok. do i need the fancy tool to squeeze the pads, as shown in the manual? and do i understand that turning the allen screw in will pull the pads away from the rotor?

also, what could be causing this? something bad with the piston? something bad with the line?

your help is greatly appreciated.

[red914]

ooooo... pelican article. i will reply after printing and reading that. thanks!

[lapuwali]

The Haynes manual is actually pretty unclear on the venting gap. Or rather, they don't really explain non-intuitive way the adjusters work.

Both adjusters screw into the piston itself. The outboard one is just a simple screw with a sealing flange and an O-ring midway along. If you "tighten" it, it screws into the piston, if you loosen it, it unscrews. Note that the screw does NOT pull the piston away from the rotor, it just goes into the piston. Go far enough and O-ring no longer seals. Leakage ensues. You have to "tighten" the screw, and push the piston away from the rotor. It obviously helps greatly to pull the pads, and get the adjustment loose (pushing the pistons with a big screwdriver works), then replace the pads and use the adjusters to push the pistons towards the rotor to set the clearance. You do this on the outboard adjuster by "loosening" the screw.

The inboard adjuster works differently, in that there's a gear in there because the adjuster isn't directly in line with the screw that goes into the piston, so the loosening direction is reversed. You "tighten" this one to move the piston towards the rotor. When moving the piston back on the inboard side, you need to "loosen" the adjuster a bit (until you hear a racheting sound), then move the piston.

Note also that the dust cap over the inboard adjuster takes a 5mm Allen, while the adjusters take a4mm Allen. The inboard dust cap is NOT optional. It keeps the inboard adjuster screw in place so it won't fall out. The inboard dust cap is often stuck on well tight, and the socket head strips all too easily. If this happens, you'll need to remove the caliper to get the cap off.

If you get lots of leaking when doing this, and it doesn't stop (esp if it appears to be leaking from the e-brake arm), then you need to remove the calipers and disassemble them. An O-ring has failed on the inboard adjuster. PP has the rebuild kit you need to replace all of the seals and O-rings.

You'll also need to re-bleed the brakes after doing this.

Do I sound like someone who just rebuilt their rear calipers?

[ArtechnikA]

this is also a classic symptom of an old brake flex hose.
they swell inside and won't allow fluid to return to the MC after the caliper is pressurised.

if cracking the bleed fitting releases some residual pressure and frees the caliper - bingo. (bleed the caliper after doing this ...)

rear brake flex hoses can be a b!tch but if they're bad, they've gotta go ...

[Bleyseng]

(IMG:style_emoticons/default/agree.gif)

[red914]

time to head down to the garage and try a few things. i will try to post an update shortly. the pelican articles were informative, yet these posts, responding to a specific problem, were a bit more so. now to see where it gets me. thanks again!

[davep]

The rubber lines can swell closed such that the MC can force fluid through the restriction, but it does not allow fluid to return back. This is a very fast way to wear pads out, so if the pads are worn more on this side it is a good indication of this problem.

The pads can rust into place and not easily move in either direction. If after removing the pins and the pad spreader, the pads are very difficult to remove, then this could be part of the problem.

Using the adjuster screws, either inboard or outboard, should move the pistons provided the pistons are not rusted in the bores. Without the pads in place, the adjusters should move the pistons smoothly and easily. If the calipers have not been properly maintained, then the adjuster mechanism that is locked into the piston could have been pulled out of its locked position, and the adjuster will not work properly. This happens when the pads wear out without the adjusters being used to periodically reset the venting clearance. A good reason to adjust say every 10000 miles.

The piston seal, the square profile O-ring, does more than seal the piston to prevent fluid leaks. Half of its function is to distort outward as the piston is forced out during braking, then to pull the piston back afterwards. If the bore and/or piston is too rusted, the O-ring is unable to pull the piston back and provide the venting clearance. Using the adjusters to set the venting clearance is just accomplishing much the same thing , but with the addition of properly positioning the spring assembly in the piston to assist with the pull back. So both the O-ring, and the spring mechanism retract the piston after braking, and the wobble in the rotor pushes the pads back.

A seized parking brake cable can also keep the brakes on. In the 914, the parking brake basicaly uses the inboard piston to push the inboard pad against the rotor. If the rotor flexes a little, then the rotor will push against the outboard pad. Thus the braking action is achieved. When the calipers lever arm is pulled, an internal cam pushes on the adjuster assembly forcing it outward. Exercising the lever should produce noticeable, bidirectional, movement of the inboard piston.

[red914]

in reply to davep:

the parking brake shows no effect on the right side, though it seems to function just fine on the driver's side. i can lever the right side parking brake at the point it attaches to the caliper, producing some movement, thogh not visible in the calipers.

the rubber hoses are only 5 months old, so i can't imagine them being the culprit.

the calipers are still shiny, as the PO had them replaced. it doesn't seem likely that the pistons would rust, when the front calipers are well weathered and seem to work fine.

back tothe garage to test the "seized parking brake" theory. thanks for the thoughts.

[red914]

well, it is "jack stand city" until i get this figured out. thanks for all the help. i may try to beg some technical assistance from some of the west puget sound gurus in the near future. looks like i am going to learn about brakes next. ah well...