Looking for 1964 F600 brake help

[iwlbcnu]

When I step on the brakes I get air sucking from the vent hose behind the seatin the cab, it does it the whole time the brakes are on, it doesn't seem like it should and I have a bad check valve somewhere. Any ideas?

[th1567]

My 85 LN900 (460 4V, 5/2 trans, 28500 GVW) did something similar...but it wasn't too loud..not annoying/worrying loud... if it runs down the road may I suggest you swing by a shop that repairs big truck..and ask them to just listen...might be normal or signs of weakness some'rs. Incidentally, I had 96 Dodge Van that had a pretty loud hiss when the brakes were depressed...In both cases, brake operation was normal empty, loaded whatever.

TH1567
We never did find out how much was too much of a load on that old Ford...
TH1567