I had a rear wheel cylinder go out and I replaced it last fall.
There was fluid leakage at that wheel.
Before I got the brakes bled, it snowed.
Winter had begun.
So, I thought about all winter.
I never found the motivation to complete the job until it warmed up, the snow melted and the ground dried out.

So, I decided to bled the brakes today.
When I use a vacuum pump at the wheel end, I get a little fluid and air bubbles in the clear tube attached to the drain valve.
When I pump the brake pedal, the pedal will slowly fade when held after pumping the brakes up.
There is no fluid leaking anywhere under the vehicle.

I'm thinking that the master cylinder needs rebuilding.
Anyone agree, disagree, or have any other ideas as to what might be at work here?

 

There are a TON of threads about this very issue. You have AIR in the lines. You need to manually bleed the brakes until the fluid is bubble free in a receiving tube / container. Until then, you'll have the brake fade you're describing.

Look under the 1st gen topics and do a search. Very common problem that can be easily made worse by not bleeding in the proper order or opening the bleeder too much during the process.

Took me forever and a few gallons of brake fluid until I figured out I was the problem...

 

Even after replacing 90% of the brake lines at one time with my truck, gravity bleeding has left me with a firm pedal with very little effort, even after opening lines on both sides of the ABS module (RWAL only). The air bubbles you see in the clear tube leading to the suction canister could just be from air leaking around the bleeder screw, not a good way to tell if you're "done" bleeding.

Either way, if you see any wet spots around the back of the master cylinder (doesn't take much sometimes) then the master needs to be repaired/replaced, does not always show though. How was the pedal before? Honestly, if you just changed the wheel cylinder it should be a pretty easy bleed job, and you probably wouldn't feel much difference without bleeding it and being careful on the install from my experience.

 

Hi NJTman,
Thanks for responding.
As I said:
"When I pump the brake pedal, the pedal will slowly fade when held after pumping the brakes up.
There is no fluid leaking anywhere under the vehicle".

Now, I have bled brakes many times before [ my shade-tree mechanic career began in 1959] and it is possible [OK, probable] that there is air in the branches to the other wheels.
It is also possible that the master cylinder may need bleeding also.
That may be why it is taking so long to get the air out.
But, to the best of my knowledge, after the brake pedal has been pumped to the point that the pedal is solid and firm, it should not then slowly drop when the pressure is held constant, as if there is a leak.
This is not mush in the pedal.
It acts and feels like a fluid leak.
But, no fluid is appearing on the ground under or around the vehicle.
This leads me to believe that it is an internal leak.
The only place that I can see that happening is in the master cylinder.
Another possible complication point is my trying to use a vacuum pump to draw the fluid down to the wheel.
I have never tried this before.
I have always pumped the pedal up, braced it, then run back to open the wheel cylinder valve.
I'm just trying to beat the running back and forth game.

@gorms
Quote:
"The air bubbles you see in the clear tube leading to the suction canister could just be from air leaking around the bleeder screw"

I hadn't thought about that.
That is possible.

I just counted up.
It's been about 6 months since I changed the wheel cylinder.
Winters are long up here.

 

Probably is the master. I think they're reasonably cheap, the other alternative is the ABS module (never seen a slow sinking pedal because of one but others have claimed it) but that would be a waste of more money then a master cylinder, my opinion.

FWIW, I tried the vacuum bleeding once and because of the air getting past the bleeder screw/hose connection I never knew if it was drawing air bubbles from the fluid chamber or what. Did not work any better. Now I gravity bleed.

 

Your description above is exactly what my truck does when there is air in the lines, regardless front or back.

Personally, I would bleed all 5 bleeders, back to front, including (#5) the abs unit, although 2nd gens may have more ?? Not sure basing my experience on my rolling brick.

It's essential to never open the bleeders more than enough just to let the fluid out, then close, then "pump em up again". If you're not bleeding the whole system, and the air has been in there for 6 months, it is possible that your master is airbound, but I've never had to bleed the master, other than bench bleeding prior to initial install (from a new box).

I have a "Vacubleeder" system that I purchased, but I couldn't get it to work properly on my truck, hence the reason I went back to manually bleeding them with a helper (wife).

From your OP, it comes across that you didn't bleed the other wheels, ABS unit. FWIW, that fluid is contaminated as when you changed the WC on that rear drum, air entered the system, and stayed there for 6 months ????

You might just be dealing with a major rust issue in the inside of your lines, as well as air that has creeped back up into the master. Follow the KISS Principle. Keep it simple stupid.

Bleed the entire truck after putting all new fluid in the master (use your vacubleeder to suck out the old junk). Get all new fluid in the system and see what happens. Worst case scenario is that the air caused contamination in your RWABS unit or master, and would have to be replaced. Best case ..... changing all of the fluid and purging the air returns it to normal..


Good luck, and keep it posted as to how it turns out.

 

I had the exact same experience.......and it's the reason I manually bleed only now.

Supposedly, pressure bleeding is the best, but I've yet to purchase a kit for doing that.

 

OK folks.
I think this is coming together in my head now.
The wheel I've been playing with is the rear wheel closest to the m/c.
That means that I'm only 1 wheel away from the point that I should start from.
That's why I wasn't too worried about starting with it.

Now, I do have a factory service manual for my vehicle.
So, I went looking for the ABS info, or as they call it in the book, RWAL [rear wheel anti lock].
I'm going to need to read through that section as I've never dealt with ABS before, except to ignore it when the light comes on.
That was the official Instructions from management on how to deal with it for the bus that I drove for work.

I have found this:
Quote:
Recommended sequence:
1. Master Cylinder
2. Combination Valve
3- Rear Antilock Valve
4. Left Rear Wheel
5. Right " "
6. " Front "
7. Left " "

From that, I take it that they use L & R looking from the front back to the vehicle.
And from that, the one I've been playing with is the Right Rear.
If it wasn't the factory manual I'd argue with it.
The LF wheel is further away from the m/c than the RF. By the usual logic LF should come before the RF.
The rule has always been:
start with the furthest away and work towards the closest.

ABS [RWAL] adds a new dimension to this.
I have something to learn.

I'm also working on another vehicle, a Dakota: cleaning out the cooling system, flushing the tranny, etc.
And, I think I'll work on cleaning up the batteries and cables on the CTD for a bit while I'm figuring this out.
Plus, I bought a code scanner for the Dak.
Out of curiosity I'm going to hook it up to the CTD to see what it says.
I may be back with more questions and will be back to report when I get it done.
Thanks guys.

 

Also, check the rubber brake lines to the front calipers, I have read they can deteriorate and swell under pressure...but would think they would not hold much overpressure before swelling.

Just a thought..

 

Yes, they can deteriorate, and they have.
I replaced both of those lines about 3 years ago when one began leaking.
They could only cause the brake pedal to act as it does if they were leaking.
I will look at them though just to be sure.

 

free, one other thing with a RWAL system that can drive you nuts is if that RWAL valve isn't seating quite right, it can do the same exact thing. It operates in the following way, normally it is just a sealed up orifice that leads to a pressurized accumulator. When the ABS senses rear wheel lockup, it opens the solenoid to allow the rear brake line pressure to bleed into the accumulator, until the rear wheels are no longer locked. You can see how this could really mess you up if either the solenoid is leaking ever so little, or the check in the fluid return is doing so.

Bleed it 6 ways to Sunday, replace the MC if you want ( don't get a reman, go with a Raybestos or other name brand new ), but if that don't do it, don't forget that little RWAL valve.

Luckily, 94-99 1/2 ton, full size van, 3/4 and 1 ton were all the same, so they shouldn't be too bad to find in the yard.