Hi! Been doing some brake work up front and shocked to find I have no pressure at rear brakes...

I have a 2001 2WD QC 4.7L AT Dakota with ~260k miles. Spent it's whole life in central NC - so not much rust... I had sticking caliper issues and recently replaced all front brake components (flex hoses, rotors, pads, calipers) and bleed front brakes... I've owned the truck for ~10 yrs and put 60k miles on it (it had ~200k when I bought)... As best as I can recall the brake pedal has always been a tad spongy - never as solid as our 2014 Toyota or 2016 Honda. Kind of chalked that up to the age of the vehicle.

Well after I replaced the front components my pedal firmness was exactly as it was before... so I went to bleed the rears and would then go back to re-bleed the fronts again. To my surprise I could not get any fluid to come from rear bleeder screws... I removed the bleeders and confirmed they were not clogged... not a drop of fluid came out of the bleeder screw holes after bleeders removed... I now presume I haven't had rear brakes for a while (years?)...

As I've researched this in general and specific to 2nd Gen Dakota's I've found multiple possibilities:
1. blockage in the line feeding rear brakes - maybe delaminated center rear brake hose as it feeds both rear wheels?
2. Two bad wheel cylinders? But in 2013 (229kmiles) I had both replaced because they were leaking, also replaced brake shoes, got drums sanded, and rear brakes bleed (this is all listed on my receipt) - so assuming the mechanic was legit, the rear brake system was working at this time. Presume could still be 2 bad cylinders but guessing not likely...
3. Proportioning valve? I've learned what it is and how it functions - so it's very possible that the 'switch/valve' inside has been activated and is blocking fluid from the rear. However both my ABS and Brake dash lights go out once the truck is started (so they are working but are not staying lite up). It's my understanding that if the 'switch' in the valve was tripped that the dash lights (ABS and/or Brake?) would stay lite (guess it's possible that the brake warning switch in proportioning valve could be faulty).
4. ABS system - believe my truck only has rear ABS. Could something here be blocking fluid to the rear?

Before I start replacing hoses, wheel cylinders, proportioning valves, etc I figured someone here might be able to eliminate some of these options... so any advice you can give is appreciated!

Thx!
Scott

 

Grab a friend, park friend in drivers seat. Have said friend pump up the brakes and HOLD pressure, while you crack open a FRONT bleeder. Doesn't matter which one. Do that a couple times, and then have said friend pump up the brakes. is the brake lite off? If it is, try and bleed the rears.

 

Side notion from HeyYou, and on your original statement - crack the rear line at the frame rail. If you get pressure there, then your rear soft line is probably collapsed. I'm surprised it would collapse dry and not locked up, though.

 

I appreciate the quick response! So when I bleed my brakes I use a MightVac... sounds like you're saying to do a couple normal bleeds on one of the front brakes - with the car running - and assuming the brake dash light goes out, try to bleed the back brakes.

I can certainly try that... would you mind sharing the logic behind this test just so I understand what I'm checking? Is it critical to use a friend rather than the mightvac? I've never bleed brakes with the car running... am I correct that you want the car running so I can confirm the brake light isn't on?

 

When I get to the point that I'm tracing the 'blockage' from back to front I would do this... I think the rigid rear brake line runs from the proportioning valve (next to master cylinder) down the driver's side frame to the flexible brake hose at the rear axle. If I'm correct then I could disconnect at proportioning valve or at rear connection to flex hose... sound accurate?

 

After work I got to do some quick troubleshooting... Cracked open the connection where the rigid brake line to the rear of the truck connects with the flex hose brake line and fluid quickly started seeping from the joint... Then cracked open the top rigid brake line (to driver side rear wheel) on the other end of the flex hose and again fluid started seeping quickly... So unless both rigid lines going to the rear wheels are blocked I presume that means I have two faulty wheel cylinders...

Does that make sense?

And here's something i find very weird... After breaking open 2 connection points in the rear brake line system my pedal is totally unchanged... In my experience with dealing with brakes I would have expected some air to get into the line and I should have felt that in the pedal's firmness. Agree?

 

Is the brake light in the dash on?

 

No sir... Bulb is good and goes out once started...

 

Cracking a line briefly generally won't inject air to the system. The brake lines are gravity-pressurized, so once you crack the line, the weight of the brake fluid pushes out, but no air will get in (again, assuming a very brief crack). That's why racecars run a residual valve with a low-mount master, to artificially make that pressure (2psi for disk, 10psi for drum).

and given your description, I likewise am under the impression that you have 2 failed wheel cylinders. So now, the next question if if you get hydraulic pressure unhooking the rigid lines from the wheel cylinders.

 

Brake fluid is hydroscopic. It absorbs moisture out of the atmosphere. Maybe the body isn't rusty but if you're near water, it can rust inside. That's why flushing brakes are so important. I'll bet when you pull the wheel cylinders, you'll find them full of rust and that's why no fluid came out. (If there's any humidity, there's water nearby._