Yup...it definitely is!
If it's as old as the boat, you'd be better off replacing the pump...it's easy to do 'cuz you only have to remove and replace 4 bolts and a couple of hoses and it's done.
But while a new joker valve will block the backflow for a while, but neither a new pump nor a new joker valve will eliminate what's causing the backflow. If it is blocked tank vent (and I'd bet real money it is), every flush will increasingly pressurize the tank, which can result in anything from an eruption in the toilet (I've seen joker valves turned inside out from the pressure) to a spew when the pumpout cap is opened to a cracked tank...none of which you want to happen!
So do NOT use the toilet again or try to pump out or dump the tank until you've cleared the vent. Without a source of air to replace contents as they're being sucked out of the tank, the pump will pull a vacuum that'll prevent it from pulling out more than a gallon or two. A particularly strong pumpout can even implode a tank.
The two most common locations for a vent blockage are the vent thru-hull and the other end of the vent line--that end of the hose and the vent fitting on the tank. Start by cleaning out the thru-hull...use a screwdriver blade, ice pick--whatever works. If that doesn't result in a spew out the vent, you'll need to relieve the pressure before removing the vent line from the tank to clean them out...so open the deck pumpout fitting VERY CAREFULLY with a hose at the ready. Be sure you're UPwind of it! Scrape out that end of the vent line and the vent fitting on the tank...replace the vent line.
If there's a filter in the vent line, remove it...once a filter gets wet--which can happen when a tank is overfilled or even due to heeling--the charcoal swells and blocks the vent...one of the reasons why vent filters aren't recommended for use on most sailboats.
I'm guessing that you're fairly new at dealing with marine toilets and holding tanks. The title of my book (see link in my signature, just click on the title) The title (my publisher's idea) is a bit misleading...'cuz although it does deal with every source of odor on a boat and how to cure, or better yet PREVENT 'em, it's actually a comprehensive "marine toilets and sanitation systems 101" manual that explains the laws, describes all the types of systems and how they work, and will help you learn how to operate and maintain your system to prevent 99% of problems instead of having to cure 'em. 'Cuz you get to do any preventive maintenance on your terms when it's convenient...the need to cure a problem never happens when it is! And I'm always glad to answer any questions it doesn't.
--Peggie