Rear brakes locking up

[mjfeeney4]

A couple of times in the last 6 months both rear brakes on my 95 525i ( 141Kmi) have locked up. It appears that the brake calipers binding but I thought it stange that both sides would behave the same way, locking up at the same time. Usually if the car sits then the pressure on the pads subsides and the car is drivable agian.

I have replaced the brake fluid at regular intervals and serviced the brakes myself. Other than rebuilding or replacing the rear calipers, any other suggestions?
M Feeney

[Bill R.]

the pedal up and the port that allows fluid back into the reservoir is staying shut. It can be a bad master itself causing this or something wrong with the pedal linkage or booster Anything that will not let the piston in the master come back all the way when you let the pedal up.

A couple of times in the last 6 months both rear brakes on my 95 525i ( 141Kmi) have locked up. It appears that the brake calipers binding but I thought it stange that both sides would behave the same way, locking up at the same time. Usually if the car sits then the pressure on the pads subsides and the car is drivable agian.

I have replaced the brake fluid at regular intervals and serviced the brakes myself. Other than rebuilding or replacing the rear calipers, any other suggestions?
M Feeney

[mjfeeney4]

One other thought, aren't the opposing (RR and LF, LR anf RF) brake calipers pressurized by separate parts of the master cylinder? This is why it is strang that only the rear brakes are locking. I'll check the master cylinder this weekend.

[winfred]

the later e34 has a 3 channel abs pump, rf lr and rear, all have two lines from the master to the abs pump. the early cars 91 and older have a 4 channel pump. when i built my e34 i had to convert the braking system to the later abs system because i used a 92 body wiring harness and it wouldn't work the early pump, and because my luck sucks the 1st pump was bad, the left front wheel would lock up and stay locked till the pressure was releaved. i wound up using a e30 pump with the electronic controls soldered onto the solonoids, because i couldn't find the right unit, here we are 2 years and 27k miles later still absing away. it's possible that you are having a problem like that

[Bimmer Nut Ed]

Why would they seemily regress from a 4 channel system to a three channel system?

Also, does the number of channels explain why american car ABS systems seem so poor. At least older systems. I remember a Ford Conversion van I owned, when the road was wet, it seemed like you would brake forever to stop it, and the pedal seemed to push back at you. In that respect, it feels like the difference between Wifey's 600$ ABS option on her 650GS Motorcycle, and my R1150 2200$ option ABS. The R1150R feels like BMW car brakes with ABS. The 650GS feels like American car ABS when engaged.

the later e34 has a 3 channel abs pump, rf lr and rear, all have two lines from the master to the abs pump. the early cars 91 and older have a 4 channel pump.

[Bill R.]

you would have dual diagonal circuits lf,rr etc..


But your setup is like this instead


Thats why i suspect its a problem with the master cylinder piston not returning far enough or the pushrod holding it too far in...

One other thought, aren't the opposing (RR and LF, LR anf RF) brake calipers pressurized by separate parts of the master cylinder? This is why it is strang that only the rear brakes are locking. I'll check the master cylinder this weekend.

[winfred]

a hydro boosted car can lock up if the power steering fluid tank is overfilled

[Bill R.]

an overfilled one too : )

a hydro boosted car can lock up if the power steering fluid tank is overfilled

[mjfeeney4]

Thanks for the the explaination Bill R. I'll look into the master cylinder as you suggest. I tend to be analytical ( most likely my chemistry training), who has to understand how things work so your diagrams and explaination are excellent.

November 12 update: The problem was the master cylinder, exactly as Bill R had suggested.