Brakes locking up LF/RR

calitoej · Post by **calitoej** » 10 Jun 2022, 20:56

Wondered if anybody had any ideas. My 2004 XC90's BCM went out 3 years ago & figured with close to 300K on her, wasn't worth it to send off to Xemodo for reman since it brakes fine, just no ABS or CC. This past week my LF wheel starts dragging & temp gets up to ~350F compared to all the others at ~130F. Cleaned/greased pins & bled brakes. Doesn't help. Pop out caliper piston & it looks pristine. Replace piston seal & boot. Replaced brake hose to caliper. Then it brakes fine for a few days then LF & RR lock up. Bought a used caliper just in case and put that one on and same thing. When I have it on my lift & put it in drive LF is locked. If I tap the brake wheel will start to spin about a quarter of a rotation and then lock again. Undoing bleeder free's them up. Seems like something is sticking not allowing the back flow of fluid. I have a used BCM coming in a couple of days since I read it might be a valve is sticking in the middle, silver portion(Valve actuator?). I'm going to pop middle portion/top abs motor/& my control module in and cross my fingers. Wondering what else I could do to trouble shoot. Could it be the master cylinder? I was under the impression when those go bad it makes the brakes spongy not lock.

Post by **pgill** » 13 Jun 2022, 17:18

The Master Cylinder has two chambers.

One applies pressure to the RF/LR

The other applies pressure to the LF/RR

The ABS module does manage both of these circuits (Which is why it has 2 inputs from the master cylinder and 4 outputs one to each wheel)

If the piston in the master cylinder doesn't retract enough to open the port to the reservoir then it will cause the brakes to stick as you are experiencing.

Good luck

Paul