The rear slave cylinder is identical on all T2 and T3 kombis from 1972 onwards. They are cheap and readily available from any VW dealer, VW aftermarket supplier, or brake repair shop. Repco or similar would order them in overnight. There are plenty of good VW shops up your way.
Last time I bought one from Tooleys Imports (Sydney) It cost me about $16 complete. At this price, it is not worth mucking around with buying a honing tool and installing new rubber cups and boots. Ask the salesman if there is a choice between cheap imitations and genuine quality, as the original German cylinder is readily available.
Buy two and replace both sides at the back.
While your at it, clean the sand out of everything as it is very abrasive. Spin your wheels while the car is on the jack-stands to check for noisy wheel bearings, and examine the rubber gaiters on the CV joints - if they are split or damaged, the sand will have to be cleaned out of the CV's to save expensive replacement down the track.
Clean and spray the handbrake cable (where it enters the conduit near the back wheels) with WD40 and clean and lubricate the gear linkages on the side of the transmission.
Jack up and check the front wheel bearings and brush the sand out of the front brakes too.
Check and clean the air filter and throttle linkage.
After driving through sand and salt water you need to do a day's work under and around the vehicle. It is best to brush or blow the dry sand out first, then use a high-pressure water blaster (be careful around the delicate rubber components).
The best way to sluice the salt away is to drive several times through a freshwater creek or river.
Allow the water to dry away for a day or two then spray liberal amounts of lubricant on everything that moves. Buy Inox, WD40 or similar in a bulk container, and use a weed-sprayer to coat the whole underside of the van. It's cheap insurance, and stops the van rusting away before your eyes.
I used to run a Baja Bug and learnt the hard way!
You'll be surprised how sweet the gearchange and everything else will work after you lubricate everything under the vehicle.
Consider spraying a rust-preventative under the vehicle if you do regular beach work. There are many available, but choose one that does not leave a sticky residue that attracts the sand.
Roger (Beetle) Bayley.