1992 TRX 300 4×4 Brakes lock up hard

Hi,
I am new to the forum. Thanks to all who post here to help novice mechanics like myself. I have an issue where my brakes will lock up extremely hard the first few times I use them. I mean, really hard. Chin hit the handlebars hard. After a few brake activations, the brakes seem to work normally, until the next time I use it. Had it in the shop, they couldn’t find any smoking guns, adjusted the rear brakes, seemed ok for a while and now it does the same thing. Any ideas out there?
Thanks,
Phil