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diagnosing brake issue - 73 270

Posted: Fri May 10, 2019 1:02 pm
by tworail
Hi folks,

I have a 73 270 with M375 chassis and dual booster setup. Last fall before I parked the rig after 1.5 years of good braking operations I had a pedal going to the floor and brake light coming on. Getting to the bottom of it now...

I pulled the master cylinder cover and found the fluid was down quite a bit. The boot was in good shape and appeared to make a tight seal. I also prior to this could not find evidence of a leak anywhere, at least easily visible. I refilled the cylinders to 1/4" from the top and put the cover back on - a bunch of fluid spilled over when this happened.

After pumping the brakes a few times (engine off) I had the original very firm brake feel. After starting it up and pumping some more it was a bit erratic with the light coming on mid stroke briefly, and getting some feedback from the pedal I wasn' t familiar with. Checked for leaks around the master cylinder but could identify none. After some time the pedal would go to the ground again, and couldn't build up the same pressure as I initially had before I turned the engine on.

Is the next step to bleed the system starting from the furthest cylinder from the master? Its possible the fluid was low enough in the cylinders that air got into the system, I can't say with certainty it didn't. I never pumped the brakes with the cover off during the refill.

Thanks all for any advice you can offer.

Re: diagnosing brake issue - 73 270

Posted: Sat May 11, 2019 6:24 am
If the fluid gets low enough air can get in through the master. there could be a small leak at one of your boosters where the fluid is being pulled into the vacuum. A weeping wheel cylinder is another possibility. A good bleed and fill and watch closely is the only advice I can give at this time

Re: diagnosing brake issue - 73 270

Posted: Sun May 12, 2019 8:22 am
by rjsdotorg
You can slowly lose a good bit at the rear before you notice it running down the tires; if you lost a whole reservoir full, it's time to pull the drums and check.
If there is none in the booster vacuum line and the power from it is OK, then it is cylinders...

Re: diagnosing brake issue - 73 270

Posted: Sun May 12, 2019 8:31 am
by rjsdotorg
As clean as they looked after a hone, those little pits caused leaks.

Order new cylinders from Ebay, these are rears (71 M375). You don't need the special lower ones, you can get 2 upper each L and R and add an extra bleeder to the ones you put on the lower.

Re: diagnosing brake issue - 73 270

Posted: Sun Sep 08, 2019 9:11 am
by tworail
Just to follow up on this - once the brakes were bled at the shop it was back to normal. Firm pedal feel and stop on a dime. Well not really but you know what I mean :-)