740 & 760 Brakes dragging - master cylinder?

BradBrad @Brad Sunshine Coast, QLD

Car is a 1990 740GL sedan with Bendix twin piston calipers at the front and ATE at the rear and ABS.

All four wheels are difficult to turn by hand. The front wheels are slightly harder than the rears but the difference is minimal. As the wheels are rotated there is a definite hard-rubbing sound from the brake pads.

I have tried removing the brake pads and the wheels then turn freely. I had a lot of difficulty retracting the pistons on the front calipers as there was a lot of resistance.

Since this is affecting all four wheels could this be the master cylinder at fault? Is there a way I could test if it is the master cylinder? The brakes seem to operate fine, this car has always had good brakes. The pedal doesn't sink to the floor.

It had new pads and discs on the front about a year ago and new brake hoses about 2 years ago. New pads on the back a couple of years ago, rear rotors are fine. It's just about due for a brake fluid flush but I doubt that the fluid itself could be the cause.

Thanks for any advice.

Comments

  • SpacSpac @Spac Canberra-ish.

    Does the problem get worse once the car has been driven and gotten some heat into the brakes?

    Has it had any parts replaced recently? Has it been sitting around for a long time?


    Open a bleed nipple and see if it 'ejaculates' some fluid out. If it does, then see if any/all wheels turn more freely.

    jamesinc
  • BradBrad @Brad Sunshine Coast, QLD

    I haven't tried it with the brakes hot but I'll be able to do that tomorrow. No recent parts replacements other than those listed above. It hasn't been driven much lately - maybe once or twice a week for the past 3 months or so.

    I'll try opening the bleed nipples tomorrow too.

    Thanks Spac.

  • I had a similar problem with a '90 740 with ABS though only when hot. Replaced the master cylinder and problem remained, no difference.

    Having said that it has been fine for a couple of long drives recently.

    If it happens again the booster will be replaced. There was someone else on the forum with a similar problem recently too.

  • BradBrad @Brad Sunshine Coast, QLD

    Update.

    Finally got around to taking the car for a drive to heat up the brakes. The wheels were still stiff to turn by hand just as when they are cold. Opening the bleed nipples results in a very slow dribble of fluid - no squirting. I don't have much time to deal with this so it may end up going to a mechanic or brake specialist.

  • Not sure if applicable, but helping a bloke replace his master, rebuild/hone calipers, and flush fluid, his dragging brakes were from swollen brake lines. Cheap enough insurance on a 30yr old car.

  • One other thing to check, if you disconnect the vacuum hose to the brake booster, do the brakes immediately 'release'? if so, you have a problem with the booster, you should then consider replacing it. (its and easy enough test).

    Another problem less common (as referred to by Alex) are dodgy degraded flexible brake lines, but you usually won't get that happening on all four brakes simultaneously...

    Good luck with it.

  • BradBrad @Brad Sunshine Coast, QLD

    Thanks @BarelyRunning and @arebee .

    The flexible hoses were replaced about 2 years ago so they should still be fine I would think. It had a leaky booster a few years ago which I replaced with a used one but I hadn't considered that as a possible cause of this problem. I'll try your suggestion and see what happens.

  • dmcROLL DMC @dmc Melbourne

    Same on mine that was dragging. New hoses moments after it arrived in my possession.

    Car is in hibernation now, when it is out again I will try to get them to bind up and try the vacuum hose removal. It will almost certainly not be the problem as once while driving on the freeway they were binding up to the point of full throttle, including boost, to maintain 90 km/h. I stopped for fifteen minutes, when the vacuum would have been atmosphere, and they were still tight after ten minutes. After the fifteen they were less tight though driveable, I just did not use the brakes for a few minutes (freeway, achievable) and they freed up a little. Eventually they went back to normal and the drive home (that day) was fine.

    Breakdown:

    1990 model

    New OEM/Brembo rotors

    New Brembo pads (would not recommend, always sqealing)

    New brake hoses (good quality, forget brand)

    New fluid

    second hand master cylinder from a 940

    Turbo engine completely stock

    ABS, again untouched.

    That leaves the booster and the ABS unit not replaced. I recall from tech when ABS was new that the ABS system on the car (which was current back then) had three systems. Two front and a rear. All it did was open a valve to release pressure when a wheel speed sensor was moving slower than its two peers - i.e. locking up or locked up. Not sure how all three of these valves could be binding up and causing all 4 wheels to bind up. I believe the new ones have pumps in them and can also apply the brakes if needed, the old ones are quite simple and can only relieve pressure.

    Oddly, the front right wheel will 'un brake' via the ABS at very low speeds too. Talking driveway speed, walking pace. Cleaned up the sensor, no change, and have a spare ABS unit for it. Wiil get to that after a few other projeccts are finished.

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