Rear brakes locking up

[Ricks72Chlgr440]

I parted a 70 318 Challenger with manual drums some years ago. If I recall correctly it had the same Weatherhead unit with warning switch, just as what you have pictured. There's not much to it, take a look at this article from musclecarresearch.com

Article-
1967 - 1975 Mopar Brake Tube Tee Autopsy | Muscle Car Research LLC
Parts-
Mopar Parts | Muscle Car Research LLC

The ones with the brake switch and proportioning valve are for cars with power discs, these had the running changes to them in the 1970 model year as pictured in this attachment. The later one was made by Kelsey-Hayes (KH).

So it would seem when they added the discs to your car they also added the proportioning valve at the rear to adjust front / rear braking bias. From what I understand, commonly done & nothing really wrong with it.

There were no manual e-body disc setups from the factory in 70 so I'm curious what master cylinder you had rebuilt - was it the original manual drum brake one?

Good article on factory master cylinders here-
Pumping Points

I'd also suggest checking for a possible internally collapsed or blocked rear brake hose, the one that connects to the rear axle.



View attachment 125684

+1 for the rear rubber brake hose. Helped my son on his '68 Fury with brake problems. Found the hose was collapsed internally and would not flow fluid properly. Another hose with brake bleeding fixed the issue.

 

[mjb765]

+1 for the rear rubber brake hose. Helped my son on his '68 Fury with brake problems. Found the hose was collapsed internally and would not flow fluid properly. Another hose with brake bleeding fixed the issue.

Was this hose not flowing at all?? My rear brakes are getting fluid, they just don't release at times until you open a rear bleeder.

 

[Ricks72Chlgr440]

Was this hose not flowing at all?? My rear brakes are getting fluid, they just don't release at times until you open a rear bleeder.

It has been long enough I don't remember for sure. Seems like it not only did not release but the flow was pitiful. We cut it open and found the inner rubber had disintegrated causing flow problems. If you check out the ideas the other guys mentioned and it still has issues I would certainly replace the rear hose. Not expensive and easy to change out.

 

[EV2RTSE]

In that youtube I posted above he mentions that sometimes the brake hose is partially blocked or collapsed just enough that fluid can still flow to the brakes under pressure / as the pedal is applied but then is unable to flow back properly when the pedal is released.

 

[Challenger RTA]

The debris in the hose acts as a check valve sometimes or all the time. Other times a bad hose will expand when applying the brakes and feel spongy.

 

[moparleo]

If you haven't replaced this hose, just do it. Not expensive and forgotten many times. And if the lines were not blownout before you added fluid there can be bits if debris in the lines, even new ones.
Just like dirt in a fuel line without a new filter.
Do a visual inspection first. Free and you can see what you are dealing with.

 

[mjb765]

I am going to replace the rear hose and blow out the rear brake line while it's open. I am also going to replace the master cylinder. I can't search 70 since there was no disc option, But I found this which looks like it will work.

Right Stuff DBMC10: Mopar Cast Iron Master Cylinder Ports are 1/2"-20 and 9/16-20 - JEGS

I just need to look at the hose in the back....it looks like they seem to include the T block...I have to look at mine.