The Stovebolt Site Forums The Parking Lot Big Bolts Master cylinder for 1966 GM 3 ton dump truck

So I own a 3 ton GM, used it for hauling grain etc. It has a 14 foot dump box, 327 engine and 4 speed Muncie. Problem is it has the hydraulic clutch and master brake cylinder in a combined unit. When I depress the clutch it goes to the floor and stays there until I physically pull it back. I have had it off and apart several times, sent it away to a brake and clutch shop and had it checked out. It worked for awhile but now is back to no clutch. The brake shop said these masters are no longer available which makes sense since most of the other trucks I have found are all manual clutches. Anyone on here run across these critters in their travels? Truck is useless without a clutch other than bump starting it in gear.

Any way you can split the two apart? I know a lot of the newer one-ton pickup manual transmission configurations use a hydraulic clutch, and are very modular in setup.

I would think that the problem lies elsewhere, is the clutch improperly adjusted or worn to the point of throwing "over center"? is there an auxiliary return spring on the clutch fork to help bring the clutch back into engagement?

I have bought some of those combined cylinders in the past, of course they are made in China. Sounds like your return spring is missing. The system is pretty simple!

Ed

My '65 C60 did the same thing with the clutch pedal. Adjust the clutch back to IIRC 3/4" free play in the pedal (I'm sure someone here will correct me if I'm wrong) the idea being that the pedal doesn't go 'over center' as @78Buckshot mentioned, before seeing resistance from the mechanism below. I did get a new one of those master cylinders at NAPA seven years ago, it wasn't even expensive. Like $78. Of course, now it lists for $139.

https://www.napaonline.com/en/p/NMCM34404

Holy moly-I found my receipt for that!! The NAPA part number is M34404 and it still shows as available on their website. While you're there, might want to pick up a spare clutch slave, too: 73466 also showing still available from NAPA on their website.

Hope this helps.

The NAPA part number is M34404 is the same as fits my 1960 C10.

Long story short, I have been through the entire clutch system, adjusted the throw out bearing linkage at the slave according to the shop manual, pulled the master off and installed a new primary cup filled it with brake fluid and still no resistance at the pedal. Push the pedal down and it goes to the floor and stays there. I have ordered a replacement unit from an online supplier so if bleeding procedures fail then I will try the aftermarket model. Also I will replace the slave as well.

Is the Clutch Fork return spring installed? Did you bench bleed the MC? Have you bleed the complete system? Do you have any leaks?

Mike B