If your breeching is properly fitted and adjusted your cart should not slip forward. I hope this doesn't offend you I have no idea how much experience you have but, are you confidant in how you hitch? Perhaps a hands on with an experienced driver would help be sure everything was properly adjusted.
I have pretty much zero experience, and asked for help when I had Spanky the first time (and had an attitude when I got replies I didn't like). But I'm in a different place now and very ready to learn. His breeching was way too low, but even after adjustment still it slipped. I don't know if it was the cart being a peice of crap and also WAY to big for him (I'll put up a couple pictures from last year so you can see what I'm tlaking about)
These are the only two I could find, I've since gotten a new computer so I'm not sure where everything is located
Both of these were pre-adjustment. The shafts are just pulled through the tugs, the lead is draped over the fence board (in no way tied) and he is bombproof times 10000 anyways (I mean really, he has spent the last year being a therapy horse for a 6 year old blind autistic boy. When I say bombproof, I mean it lol)
Anyways, I'm planning on doing things the right way this time. He is insanly bombproof and very patient but I don't want to ever give him a reason to get fed up. A little back story:
I got him last July, and he had had 60 or 90 days (can't remember which) with Patty Cloake. I spent MONTHS ground driving him everywhere, into buildings (its a small town, you can get away with a lot), in a parade, EVERYWHERE. I bought the cart shown above for $40 (I should have known it was crap lol) beginning of October. Spent 3+ weeks hooking him up and driving from behind the cart. I had had some issues with the bit (it was rough), so I had purcahsed a new one, but didn't have it yet. On Halloween my g-ma told me to bring him over, because she hadn't met him yet. Well, I didn't get around to it until late, and was in a hurry so I just drove him over there (first time with a person in the cart, it was only 1/2 mile or so). Because I didn't have a bit, we drove in a halter. He had kids in costumes running up to his head in groups to pet him and adored every second of it (tried to follow them a couple of times). All was fine, and I realize that it could have gone the very opposite way. No worries, like I said, I plan on doing things the right way from now on lol
This time, I am buying a new cart, an EE from Ozark, so I know that it will fit him and have all the needed peices.