Rear drum brakes locking up

[mrelxr]

When i purchased my 1970 mach 1 it had been sitting for 28 years, because of that many issues have resulted. One of those was the brakes. I have replaced the master cylinder and rear wheel cylinders. Front calipers were fine and stripped and cleaned the rear brake assembly. Flushed the whole system through and bled the brakes. Took the car out for a test drive and found that the rear brakes lock up when you hit the brake pedal with moderate pressure. If you brake gently its fine but under normal driving conditions both rear wheels will lock up. If anyone has had this issue with there's or know what might be the issue i would appreciate any assistance, thanks.

[non member]

Originally posted by mrelxr
When i purchased my 1970 mach 1 it had been sitting for 28 years, because of that many issues have resulted. One of those was the brakes. I have replaced the master cylinder and rear wheel cylinders. Front calipers were fine and stripped and cleaned the rear brake assembly. Flushed the whole system through and bled the brakes. Took the car out for a test drive and found that the rear brakes lock up when you hit the brake pedal with moderate pressure. If you brake gently its fine but under normal driving conditions both rear wheels will lock up. If anyone has had this issue with there's or know what might be the issue i would appreciate any assistance, thanks.

Is the wire running to the switch on the metering block near your master cylinder? If so is the brake light on in the dash?

I ask this because if you have done that work on the car & then bled the brakes by pumping the pedal to get the fluid through, then you have moved the piston in the metering block so that it blocks the fluid getting to the front brakes.

The fix is to take the switch (white plastic with 2 wires) out of the top of that block & get a fine pick & move it back to the center.  To tell if it is in the center shine a flashlight down the hole & you should see a recess for the pin of the switch to sit in.

[mrelxr]

Hi, kerry. Thanks for the reply, before i did any work the warning light was on, after i did the work and bled it the light did go out. I have pulled out the pressure switch but i didnt put a pick in there to see if it was centered but with the light out now id say it would be centered but ill take it out and check anyway. The only thing is the switch is on the side so might be a bit hard to see, also the front brakes are working but maybe not getting enough pressure.

[non member]

Originally posted by mrelxr
Hi, kerry. Thanks for the reply, before i did any work the warning light was on, after i did the work and bled it the light did go out. I have pulled out the pressure switch but i didnt put a pick in there to see if it was centered but with the light out now id say it would be centered but ill take it out and check anyway. The only thing is the switch is on the side so might be a bit hard to see, also the front brakes are working but maybe not getting enough pressure.

Yes it was a safety thing, if you blew a line or seal, the force would push the piston across blocking the other side. However it does not block it all the way & was designed to stop you ONCE that senario had happened.

When I do the brakes, I do the lot as crap has gone through the whole system. So I blow the lines out, get the master rebuilt or replaced, booster rebuilt (always need doing regardless), calipers rebuilt & new pads, disks machined or replaced, new rear wheel cylinders, new rear linings, skim the drums. I also completly change the fluid every few years as the fluid in the calipers & wheel cylinders remains static (building up contaminants).

[mrelxr]

I pulled the switch back out and with a mirror and light looked inside, i really cant tell if the piston is centered bit like i said with the light out id say it is. Im thinking it must be something to do with that metering block as everything else has been changed or cleaned out. If anyone else has an opinion id like to hear it, thanks.

[BLKPNY]

Kerry, when you say change the fluid, whenever I need to bleed brakes, or flush, I suck the fluid out of the reservior, and fill with a different colout of DOT 4 fluid, then bleed them until I get the new colour thru. Is that similar to what you'd do, if you weren't pulling all the components off the car?

[non member]

If when you look inside the hole (once the switch is out) & can only see a brass piston, then it has moved. Buy working out which side feeds the front brakes you can move it the opposite way until you see a groove in it. Centre the grove in the hole, then reattach & allow the brakes to drip bleed (do not pump the pedal).

[non member]

Originally posted by BLKPNY
Kerry, when you say change the fluid, whenever I need to bleed brakes, or flush, I suck the fluid out of the reservior, and fill with a different colout of DOT 4 fluid, then bleed them until I get the new colour thru. Is that similar to what you'd do, if you weren't pulling all the components off the car?

That works well & is how I do the cars I own & service.

[mrelxr]

Now that you say that, when i looked in that hole all i could see was a brass piston but no groove, how big a groove is it, is it very noticible?

[mrelxr]

I think i may have to remove it from the car as its pretty tight to get in there.

[non member]

The groove is the same width as the pin on the switch you pulled out.

[mrelxr]

Removed the metering block and stripped it down, it was filthy inside, so ive cleaned it and re-assembled it. Ill fit it tomorrow and see how i go. As far as that groove was concerned it was in the middle.

[non member]

Originally posted by mrelxr
Removed the metering block and stripped it down, it was filthy inside, so ive cleaned it and re-assembled it. Ill fit it tomorrow and see how i go. As far as that groove was concerned it was in the middle.

Good, while you have it out, blow the lines out with compressed air, or suck it out with a brake bleeding vacuum pump. That way your lines will have clean fluid.

Another thought, what sized rear wheel cylinders did you reinstall? If they are smaller than what was in there they will operate faster, locking the brakes up.