Rebuilt rear calipers and the fronts start locking up..Bill help!

imaircooled · 07-28-2010, 07:45 PM

Ok, I drove my car to work a couple months ago and on the way home the driver's side rear caliper slowly pushes the piston out with such great force that it completely stops the car. I can't move it even with the engine. So I get the car on the side of the road and it cools off and I literally hear it make a "ting" sound and the caliper lets go of the rotor. So I search around and I find that I need to rebuild both rear calipers.

Fast forward to about a week ago. I finally get around to rebuilding the passenger's side rear caliper. I buy a rebuilt caliper for the drivers side. I also get the driver's side rotor turned. I install braided brake lines on the rear. My wife helps me bleed the brakes starting with the passenger's side rear caliper. She pumps and pumps and it blows and squirts just barely. I add brake fluid up front and she tries it again. Finally the fluid really starts coming out. So she holds the pedal down and I tighten the bolt. I do this same thing on the driver's side rear and all seems good. So all of that is finished tonight and I'm ready to drive my car for the first time in two months. I get just around the block and the car starts pulling to the right. I smell hot brakes and I limp home. I get my car in the garage and the rear rotors aren't hot but both fronts are really hot. They are smoking a little and the wheels are even hot.

I will let everything cool down tonight and jack the car back up and take a look. What would cause the fronts to do this? Did I put too much brake fluid into the container up front? Is that possible? Any other ideas?

I'll post what I find tomorrow. I'm down because this was the last thing I needed to do to get my car ready to drive to the Porsches 2 Oxford show this Saturday.

Thanks in advance,

Flieger · Flieger's Garage

Did you replace the front brake lines? Kind of sounds like old rubber lines swollen shut. Hope you can make your show.

imaircooled · 07-29-2010, 04:09 AM

I didn't touch the front at all. Your idea is pretty logical. I have braided lines for the front so I will try that tonight and see if that's it. I hope so. Thanks for the idea.

By the way, the car is a 74 911 with M calipers.

Thanks again,

Walter_Middie · Walter_Middie's Garage

Chris,

Quote:

driver's side rear caliper slowly pushes the piston out with such great force that it completely stops the car

I'd definitely change the back ones too. It may be that one of your initial problems was an obstruction in the brake line.

Quote:

I have braided lines for the front

I don't like the braided ones as you can't see when the brake line is old or damaged. If you haven't changed the rear ones yet, they are past due.

imaircooled · 07-29-2010, 06:17 AM

Rex,
The rear calipers have been rebuilt. I also changed the rear lines to braided. So, the rear brakes should be good now. I might not have bleeded the brakes all the way. I'm just not sure why the front brakes would now be acting up.
Thank you,

Bill Verburg · Bill Verburg's Garage

there are small ports in the m/c which over time and especially when older systems are disturbed can become blocked. These systems are 30-40 years old and may have all sorts of gunk floating around in them. I suspect that when one caliper is as bad as you have described the others and the m/c aren't too much better. It would probably be a good idea to r/r everything, flush the lines too while it's all apart.

Jdub · 07-29-2010, 07:05 AM

Bleeding brakes stresses seals and the system beyond what the road can demand. I suspect you have the usual "one way" flow going on in your old front lines and a seal and/or particulate matter in the master cylinder that is preventing the fluid from fully returning - instead the brake caliper pucks remain pushed out.

Your story is a perfect lesson and do not mean to use this as an example, but when approaching this critical system consider all areas. For example, skimming one rotor only, or replacing only the rear calipers and lines means you will need to remember what you did so that you know what you still need to do when this comes up again - and it surely will.

My advice is to rebuild all calipers, turn all rotors unless they are past point and you need new, and finish replacing the lines. If you have the time and money, I would also say replace the master cylinder if it as old as I suspect. The "while you're in there" might also mean a R&R on the front wheel bearings as well.

In other words, a complete brake overhaul for a system that appears to be fairly old. Piecemeal work is not going to produce the brakes this car was born with - the difference is really quite amazing (having done this some years back on my SC). Best of luck and let us know how this pans out.

Jesset100 · Jesset100's Garage

I think the problem is the rear brake pads are not being bedded in properly. Why, the fronts are bedded so they are doing all the work stopping the car. I think to do it proper you need to do the front also then bed four corners the same time. IMO

imaircooled · 07-29-2010, 11:46 AM

Jdub and Jesset,

I hear what you guys are saying. Tonight, I will turn my attention to the front brakes too.

What do you mean by "bedded" properly?

Also, the more I search and read here the more I think I may not have bled the brakes all the way. Would air in the lines cause something like this?

Lots to do when I get home tonight.

Thanks again,

Flieger · Flieger's Garage

Bedding brakes: New Pad Bedding/Break-In

It burns off/fully cures the resins holding organic pads together. It cleans and then burnishes the friction surfaces and ensures a good mate between them. Makes the brakes work more smoothly, strongly, and silently.

I do not think it is air in the lines. That would make a squishy pedal that would pump up eventually, not clamp down and not let go. That sounds more like the rubber lines. I do not know if air could make the fluid boil somehow and build up pressure? That would go out the vent on the reservoir, though.

Jdub · 07-29-2010, 12:35 PM

Agree on the air - this is not the problem. Your old rubber lines are swelled shut and are preventing the proper return of fluid. You could wait on bedding the pads (did you replace all four; rear end?) but waiting with brakes is a dodgy proposition.

Be absolutely sure you are OK with those brakes after putting in the new front lines and rotors & pads (if rotors are under spec./appear very ridged). You do NOT want this car coming to a halt on you far from home.

As a rule, fresh or newly skimmed rotors require new pads. Failure to do this causes the rotor to wear to the ridges in the old pads. However, new pads to new rotors never feel really great (canting of the pad-to-rotor) until they all bed in together. Once that happens (and all the da#n air filters out) you will have your nice hard pedal.

imaircooled · 07-30-2010, 06:01 AM

Last night I bled the rear brakes again and then replaced the front brake lines with new braided lines. It was getting late so I cleaned up and went to bed. I'm hoping this solves the problem. I will bleed the fronts and take a test drive. If not I will then rebuild the front calipers and check out the master cylinder.

Thanks, as always, for your advice.