The Stovebolt Site Forums The Parking Lot Big Bolts Mstr Cylinder - maybe?

Ok after sitting a couple of weeks I have no brakes in my 1955 6100 dump truck. Checked the mstr cylr and it was dry. I checked brakes lines and wheel cylinders and see no leaking. Refilled the mstr and pumped like heck. Never got any pressure and does not seem to be gushing out. In looking underneath i see some dampness off to the side of the Mstr cyl. Any thoughts as to which way to go. Also were can I find a master cylinder if needed or should I just rebuild the old one with a kit from NAPA?

if the M/C was dry you inadvertedly added air to the system when you started pumping, which could be part of it. did you loose more fluid after refilling? disconnect the vacuum line from the hydrovac. if fluid pours out that your problem. pull the plug on the can of the hydrovac if fluid pours out that your problem. if it lost that much fluid from your M/C would'nt be damp it would be wet with a spot on the ground/floor. start it up and see if pretty blue smoke comes out the exhaust pipe if so its the brake fluid. i blew one one time and i thought i lost the engine. them hydrovacs can hold a lot of fluid before you figure out where its going.

Don't start your engine IF you use silicone brake fluid, and suspect it has been sucked into your hydrovac. I have heard that silicone brake fluid, when combusted becomes an abrasive in you engine cylinders...

Bigdonn,
There is a seal in the hydrovac, that if it fails, brake fluid will go into the vacuum chamber. You have to fill up the chamber before you start sucking brake fluid into the intake manifold. That chamber is pretty big and will hold several refills of your master cylinder resevoir. Thus, it becomes a "black box" for brake fluid, making you wonder where it's going. The key is watch your master cylinder resevoir level and if it gradually drops, without any external leaks, that is likely your problem.

I would be checking the hydrovac, before tearing into the master cylinder. And yes, you probably sucked air into your lines, and you need to bleed your entire system. How do I know all this? Been there!

Kent

Sorry to hijack the thread but my questions are in line with Lightholders. I just had my 55 6500 delivered and there are no brakes either. Filled the MC and it is pouring out the front. Before I get to the Hydro Vac I want to replace or rebuild MC. Does anyone know the part number or where to get a MC? I assume they are different from the pickups?
I don't see fluid leaking anywhere else such as wheel cylinders or brake lines. Is this how I should proceed?
Thanks for letting me but in.
Brian M
55 6500
59 1/2 ton Napco
59 3/4 ton Napco

All good advise here.

Brian, that is the way I would proceed.
Believe I got my rebuild kit from NAPA for my 58 1.5 ton. It is 1.25 bore as I recall. Yours may be larger, so unless someone advises you on what you need, I suggest removing it to make sure.....you have to do that anyway.
Good luck.

Don't know if this fits your problem or not, but I spent a frustrating several months trying to get brakes on my '46 two ton. An old timer told me the following as a way to isolate the problem and find out if the master cylinder was the culprit: He said to get three vise grip or gripping pliers and a supply of those sticks that you use to stir paint. He said to then clamp the flexible hoses leading to the wheel cylinders. Use pieces of the sticks to cushion the hoses so that you don't damage them when you clamp the hoses shut with the pliers. You need two for the front wheels and one will work for the back on the flexible hose located before the hose splits to feed the back two wheel cylinders. Then fill the master cyclinder with brake fluid and pump it, bleed if necessary to see if you can get pedal. If you cannot the master cylinder is bad.

I replaced mine with a rebuilt one which as I remember I was able to get from the usual suspects, Obsolete Chevy Parts, Jim Carter, American Classic, etc. Finally I was able to stop my old bolt!

I had same issue, no brakes all of a sudden.
And at the same time another colour on the exhausts. Scared the hell out of me then, but I read about sucking fluid here, so I calmed down. Also had a leaking master cyl in the front of it.

So I just ordered a new master cyl from Alretta Truck Parts, and a factory rebuilt hydrovac, and followed the instruction Geoff fixed to remove all air.

Brakes are superb. Just waiting to blow a wheel cyl now....

The big trucks have a master cylinder with a 1 1/4" bore. The last one that I got came from NAPA. I would caution against pinching off the brake hoses. Many times once they are closed off they will not come back or they will let fluid into the wheel cylinder and won't allow the fluid back out, acting like a check valve.
Don

Just received my new MC with 1.25 bore. Checked my wheel cylinders and all look great, no leaks. Opened up plug on Vac cannister and there was no fluid. Once I re-install the MC what is the process for bleeding the brakes and purging the system of air? I've never bled a Va assit system.
Any advise is greatly appreciated.
Brian M

I'll pre-qualify the following with I have never dealt with a master cylinder like yours but all that I have dealt with could and should be bench bled before installation. Just google it for directions.

after the MC is bled then you bleed the hydrovac first with the two bleeders then the wheels after your done the wheels go back and do the hydrovac again.

when you disconnected the vacuum line from the hydrovac did any brake fluid come out there?

No fluid from the vacuum line. Thanks for the help. Hopefully I'll have brakes by the end of the weekend.