Master Cylinder... master ba...

[Agnem]

Did ya take the bait?


Anyway... I've been fighting a problem with the Scarlet Moose since we put it on the road, which is it thinks it's a toyota. Kinda hard to stop. The brakes have been iffy since day one, and so far it has new calipers and pads up front, and the back checks out fine. When I took it in for inspection, they replaced the master cylinder, put the pedal was always spongy after that, and there is not enough to be able to stop very hard. So I took the master cylinder off yesterday to replace the booster, and when I got it back together the pedal just goes to the floor and there is zero brakeing effort. When I look in the resevoir while the pedal is pushed, there is a gyser of brake fluid that comes up as the pedal nears the end of its travel. Is that normal? I loosened the lines where they attach to the master and I can't get fluid to do much more than drip at that juncture. Right now it's not driveable, and I'm at a loss. Does this sound like I had a bum master cylinder from the get go or is it something else?

 

[Dsl_Dog_Treat]

What's the vacuum gauge looking like with it hooked up into the vacuum system Mel?

Is the pushrod on the booster adjusted out long enough?

I know the 83-86 master cylinders are a PITA to bench bleed but they will shoot like a whale when bench bleeding.... Must be why they say to put the cover on when bench bleeding them.


If so, sounds like somebody forgot to rebuild a master cylinder after it got cleaned......

 

[typ4]

Sounds like the pushrod is too long and the master is not filling with fluid, some are adjustable, what I do is shim the master out with washers to test then grind or adjust the rod to match

 

[icanfixall]

Mel... Is this the bendix twin chamber cast iron looking mc. If so they do have a bypass at the end of the stroke. It might be just as Russ posted and you are going past the end of the effective stroke. Which chamber is shooting the gusher... The front or the back.

 

[franklin2]

The pushrod coming out of the booster is adjustable, and there is a spec on how far it comes out in the factory manual. If it's too long, the problem it causes is the brakes start applying as you drive down the road, because the little holes in the bottom of the master reservoirs(called compensation ports) get blocked by the master being applied slightly by a too long booster rod. The fluid warms up and expands as you use the brakes, and the fluid needs to expand back into the reservoir.

In your case the first thing I would do is pull the master and put it in a vise on the bench, and either use the plastic plugs that come with new master cylinders, or use the plastic fittings and rubber lines the older master cylinders came with, and bleed the master. Each time I get a rebuilt master cylinder I save those little plugs.

I like the plugs better, and you would be surprised how many times you have to pump the master on the bench before you get all the air out. When you get it all out, the master cylinder will not budge at all when you manually push it with a rod or screwdriver. I sometimes have to pump it up to 30 times before all the air is out.

 

[oldmisterbill]

Just a thought -the symptons of a bad rabs valve are almost identical to a bad mastercylinder.I went thru 3 mastercylinders and a shop even put one on while I was on the road.Finally iin desperation I bypassed the rabs valve and the brakes came back strong. I plugged the lines on the mastercylinder while it was on the truck and the peadl was hard -thats when I decided to bypass the rabs valve. I need to put one back on since it is never hooked to the gooseneck (sold it)anymore.

 

[torinoracer29]

Just a thought -the symptons of a bad rabs valve are almost identical to a bad mastercylinder.I went thru 3 mastercylinders and a shop even put one on while I was on the road.Finally iin desperation I bypassed the rabs valve and the brakes came back strong. I plugged the lines on the mastercylinder while it was on the truck and the peadl was hard -thats when I decided to bypass the rabs valve. I need to put one back on since it is never hooked to the gooseneck (sold it)anymore.

thats exactly what happened to me and I also bypassed the rabs valve and have better brakes than before.

 

[LCAM-01XA]

Torinoracer, do you swap ends when you floor the brakes on rain or snow? I've done 360s in the middle of Telegraph road like that, full line pressure to the rear of a hydroboosted dually can make for some rather interesting experiences... Bypassing the RABS is fine for testing purposes only, but it's actually dangerous to just drive around like this as these trucks have no proportioning valve - if you don't want the RABS then install a manual brake bias valve in its place, or a factory-style prop valve, but unless you hook up to a heavy trailer all the time you do need to drop the rear line pressure.

Now as far as actually bypassing the RABS, there's no need for physically disconnecting the lines to it and joining them - instead what you can do is just unscrew the big cap in the front of the RABS unit, pull the spring out, and install a bolt of suitable diameter and length in its place, then screw the cap back on - this way even if there is an internal leak past the dump valve the piston in the accumulator chamber cannot move resulting in no fluid loss and spongy pedal.

 

[Agnem]

Everybody who said it was a pushrod problem gets a star. Turns out I grabbed the wrong pushrod and it was way too short. Put the right one in and things started changing for the better. That and a new left rear wheel cylinder fixed all my woes with regards to the brakes. Thanks everyone.

 

[icanfixall]

Glad you have brakes now... Tiss a good feeling...

 

[jwalterus]

wish mine was that easy, I have a big ol air bubble in my rear line, spongy pedal and with the top off the master you pump it and then wait a few seconds after you let off them for the geyser

only problem is the rear bleeders are rusted in place, been soaking them down with break free every day for a week and they are still stuck.... might end up paying a shop the $40 to bleed them for me at least if they break it off they have to fix it

 

[93turbo_animal]

wish mine was that easy, I have a big ol air bubble in my rear line, spongy pedal and with the top off the master you pump it and then wait a few seconds after you let off them for the geyser

only problem is the rear bleeders are rusted in place, been soaking them down with break free every day for a week and they are still stuck.... might end up paying a shop the $40 to bleed them for me at least if they break it off they have to fix it

But your still gonna have to pay for the parts. If you can get the brake lines loose just bleed it that way

 

[MIDNIGHT RIDER]

only problem is the rear bleeders are rusted in place,

Everytime I have reason to be under any of our trucks I ALWAYS squirt the bleeders good with whatever knocker-loose I may have handy at the time.

It is also a good idea to just loosen and re-tighten them at least twice a year, just to keep them from rusting up.

So long as you just loosen/re-tighten, without pressing the pedal, no air will enter the system.

It is really a good idea to go ahead and do this before actually having brake work or brake problems, so that you can go ahead and address the situation and then know that the bleeders are going to work when you need them.

I always coat the threads of any bleeder I may have out, and those on new components, with good old anti-sieze.

 

[LCAM-01XA]

only problem is the rear bleeders are rusted in place, been soaking them down with break free every day for a week and they are still stuck.... might end up paying a shop the $40 to bleed them for me at least if they break it off they have to fix it

Dude, rear wheel cylinders are like $12 a piece, just replace them old rusty suckers and use some anti-seize on the bleeders of the new ones. And if the lines on the axle break in the process, that just means they've been ready to go for a while now even if you have to get new ones, you're still under $40 for everything, only now you know you'll be good for years and years to come.

 

[MIDNIGHT RIDER]

Everytime I have reason to be under any of our trucks I ALWAYS squirt the bleeders good with whatever knocker-loose I may have handy at the time.

I forgot to mention that I also saturate any brake-line fittings, both at the threads and also where the line enters the nut.