Rear brakes locking up

[Da_vinci732]

I have a 1985 300d car drives fine but out of the blue the brake pedal gets stiff and rear brakes start dragging. But if I open the bleeder screw the piston goes back fine no problem and drives good after that. I change the brake hoses on the rear previous owner did 2 new rear calipers few months ago cleaned that valve that's on the vac pump what else could it be

[Sugar Bear]

Symptoms of hoses but you renewed them. If the sticking is due to pressure vs. mechanicals I'd guess the master binding internally. Check the linkage at the top of the pedal first for binding.

Did this start after any brake related repairs?

Good luck and keep us posted!!!

[Da_vinci732]

It started before I did the 2 rear hoses and cleaning the check valve on the vac pump

[97 SL320]

Make sure the brake pedal isn't being prevented from fully releasing. You should feel a bit of free play.

When the brakes get stiff, back off the master cyl to booster nuts and see if the brakes release. Leave the lines attached when you do this and be sure to pull the master away from the booster.

[Diesel911]

Let the brakes stick again. Block the Wheels, Jack and safely support the Car so that the rear wheels are off of the ground and the Trans is in neutral

Open the hood and loosen the Front Brake line on the Master Cylinder and see if brake fluid comes out as if it is under pressure. If that happens something inside of Master Cylinder is holding in the pressure. (Memory wise I believe the front portion of the Master Cylinder goes to the rear brakes)

[ILUVMILS]

Over the years I've seen faulty master cylinders cause this problem on 123's...….

[Maximan1]

Originally Posted by Diesel911

(Memory wise I believe the front portion of the Master Cylinder goes to the rear brakes)

That is correct.

[t walgamuth]

If the front brakes are sticking and not effective you will lock the rears.