I never had any leakage issues with Shimano and Magura hydraulic disc brakes!

If you want something that folds almost as small as the Brompton, also tri-fold but different, faster and much more comfortable, you might consider the Riese & Müller or Pacific Cycles Birdy with 10s derailleur.

Mint has also a copy of the Birdy but with a different stem and probably closer to the Birdy 2 than the current Birdy 3.