I have a 308 QV with a strange brake problem and was hoping you folks might help During very hard braking in a straight line- the front right tire locks quickly. During normal braking- everything feels fine with no pulling. Below is what has been done to the car in the last few weeks: rebuild all ATE brake calipers, piston seals and dust seals only new stainless wrapped flexible brake lines- no leaks I can see rotors turned (4) all new fluid (DOT 4 Valvoline SynPower), bled system with a pressure bleeder new pads- Axxis delux This problem was there before AND after I did the brake work- that's the really weird part. I was thinking maybe the master cylinder is weak. But the pedal feels fine except for some extra travel that is seeming to go away as I drive more. I need to look closer at the front hard lines for kinks or anything. Maybe the proportion valve? What else is there in this system that could go wrong? Thanks in advance- jwise
Gentle pressure causing a gradually sinking pedal indicates the M/C has gone (stomping on it gives a firm pedal). Have this? RF is lightest wheel (has least weight on it) and under heavy braking will lock before LF. Normal. Proportioning valve controls the F -> R distribution with a fixed (0.7) ratio. If you found the rears locking up before the fronts (not good), you'd know it and be scared by it real fast. Philip
Philip- The pedal feels fine- no sinking under gentle pressure. Now that I think about it- I'm not sure how this problem could be the MC since it does not have seperate zones for left and right front. I don't see how this could be normal performance- it would be dangerous in a panic situation. When it locks, it pulls to the left- oddly enough. The front left doesn't eventually lock. I'm thinking there must be a reason the left front is not getting the same fluid pressure as the right.
John, This has been naggingly familiar, but I couldn't remeber where I'd run into something like it before. Finally it came to me. Grant's grey '86 Rx7 would do the same thing. I checked out both calipers, pistons moved freely, no obvious problems. Swapped in the porportioning valve, no change. Rebuilt the M/C, no change. I finally threw in the towel & took it to a shop w/a couple of very good Mazda trained techs. They both decided that the MC was bad, even tho I'd rebuilt it. Choked & spent $450 for a new m/c. No change. They threw up their hands, didn't charge any labor, but since the m/c sas special order, we were stuck with paying for it. I finally pulled both calipers off. Turned out on the side that did NOT lock up, an inner & outer piston were cocking sideways & sticking under pressure. Rebuilt a spare caliper I had & no more problems. HYPOTHESIS: I suspect you've got a caliper with a similar problem. Most likely the cylinder is worn just enough so that one piston cocks slightly sideways under pressure & sticks. Only other thing I can think of is that the caliper that doesn't lock up is improperly shimmed so that the piston's aren't perfectly perpendicular to the rotor, & the rotor centered in the caliper. This could do a couple of things: 1) Piston can't produce force across the entire face of the pad. 2) Piston is cocked sideways & drags/jams as above. BTW,did this recently appear or has your car always had this problem?
Hmm- sounds logical. During the caliper rebuild, all I did was remove the pistons, clean the walls of the cylinder with a scothbrite pad (all eight cylinders had a little corrosion and I could not get it all off), install new piston seals, and new dust seals. I wonder, like you said, that there is still something wrong in there. Maybe one passageway is slightly clogged so the fluid only gets full force on one piston? It also seems weird that it pulls to the left when the right front locks. How did you determine that a piston in the RX7 caliper was getting cocked under pressure? Improper shims? I certainly could check the distance from the caliper to rotor, but where is the best place to do that? Where is there a good machined surface on these calipers? This problem started last summer- at least that's the first time I noticed it. There is one particular stretch of road on the way home from work I almost always use. At one point, there is a good place for heavy braking from about 40 mph. So, I'm driving this road last summer, and the thing locks up where it hadn't before. Thanks for the help jwise
I pulled the caliper apart & could barely see where the piston was slightly marking up the caliper when it was extended. Wasn't much to see, had overlooked it before. Also there may have been faint marks on the piston. In this case, it may also have been corrosion related, corrosion had built up on one side of the cylinder more than the other. Hmm, Since this is a relatively new problem, I it's wear or corrosion related. I'd get a brake cylinder hone & hone the cylinders to make sure they're truly smooth. If there's any uneven wear, it'll show up after honing. As to checking the caliper-rotor alignment, Since you've got new pads, pull them & look to see if the full surface is wearing evenly. Look for one edge wearing slightly more than the other. Use a micrometer to measure the pad thickness at each edge to see if the wear is uneven. Little things like that. Be sure to mark the pads when you pull them so you can put them back in the same slots, otherwise you'll have to start the bedding in process all over. When you have the pads out, you can measure the clearance between the machined inner edges of the caliper slot and the rotor. The edges must be parallel. It's desirable that the rotor be centered in the slot, but a few thou off-center will be accommodated by the pistons.
Hey John: Somewhere I have the old 308 F calipers. Sitting in the basement, gathering dust when I last saw them. Need clean up, seals etc, let me know if you need. I no longer need them. Philip
Verell, Remember i had right braking problem and we discovered that my wheel bearing was bad when we changed calipers. Perhaps he has a bad bearing?
Hi Steven- My front wheel bearings are new- but is worth a look to make sure all is torqued correctly. Thanks. I need to talk Verell into lettting me borrow your old front calipers so I can run a test. jwise
jwise, Interested to see your thread on 308 brakes. The brakes on my 308gt4 are a long way from what would be considered state of the art these days and the possibiltiy of one wheel locking up early is not unheard of. However,I had the same problem and in my case I traced the cause to an incorrectly torqued wheel bearing. In fact the lock nut had worked just a tiny bit loose. Not a very good locking system in my opinion! I can only assume that this was allowing the disc to run out of true and snatch on breaking. Tightened it up and all was O.K. If it's not the above causing your probs then the only other thing I can think of is air in one of the front pipes. Try bleeding again. Also check tyre pressures. Good luck in sorting the brakes, as I said this is only my experience with my car and not necessarily what your problem is. But it's worth investigating. Brakes are a major safety issue so don't take chances, I know how scary the lock up is at speed! You've obviously done some good work on your breaking system so you know all that stuff anyway.... Lance.
Thanks Lance- all good stuff. I agree- I sure would rather have a castle nut and cotter pin in there. I'll check them again. I'm also thinking it might be useful to change nothing but flip the front wheels/ tires. They are directional- but shouldn't really matter for a quick dry pavement test. Thanks again
