Limited heat output

[Hasty]

I have an issue with the heat on my 1970 Toro. When I push the lever to Hot, I get heat for about 15 seconds, and then it is cold again. I push it back to cold, then to hot, and I have heat again, for 15 seconds. Do I have a vacuum leak issue?

I replaced the "heat control valve" (back rh side of intake manifold, heater hose and vacuum hose hook up to it). I can hear the vacuum "phoosh" every time I switch it to cold. Also, all of the air flaps under the dash work (air directed to windshield when on defrost, floor when on heat, etc)

Do I have a vacuum leak issue? Do these switches default to cold (spring pressure?) when there is deficient vacuum?

[69W34]

Is this a Comfortron system or custom AC? If it is the latter the heat valve will have only 'one' vacuum port, which operates (shutting the valve off) only when the max AC is pushed in position.
In normal AC when the button is pushed hot water is allowed to circulate through the heater core, the heat control lever operates an internal door controlling the desired amount of heat.
The only default condition when vacuum is lost, the system will default to defroster position. Also a vacuum loss of that magnitude would be noticed in rough idle and poor engine performance.
Given all of that I would be looking at water flow or lack there of through the heater core.
As it is when you 'close' the heat door it stops the flow of air thus by passing the heater core allowing it to 'catch up', then when you re-open the door the air flow across the core dissipates the heat so fast it can't keep up let alone recover.

[Hasty]

Thanks for the response!

It is not the Comfortron, only one vacuum port. The water flow issue was one that I had considered, but wanted more information on the vacuum controls involved before I barked up the wrong tree. I don't have overheat issues (that I know of - no temp guage, just the light, but doesn't boil over or have other signs of overheat) so was reluctant to check the water flow side, but I guess that's the way to go.

Thanks again for the response.

[toro_mike]

From Darwin:

i don't necessarily think he has a massive vacuum leak, I think he has a small leak, going to the heater control valve. So he gets a brief burst of it working, then the valve reverts to off and no heat.

[69W34]

That is a consideration ... however the heat valve is always open or 'on' until the this AC is switch to Max AC at which time vacuum is applied closing the valve. This is part of the 'dealt' system if for any reason you lose vacuum the valve will revert to open and 'doors' revert to the defrost position. My money is on some sort of restricted water flow.