'95 Volvo air problem

I have a 1995 Volvo 3 axle with sleeper. Without a trailer attached, I release the brakes and I get air leaking out of the relay valve under the 5th wheel. I have replaced both air cans on the front axle, the relay valve and another quick release valve that had air lines going to the brakes on the front axle. Wondering if anyone else on here has other ideas to what it might be. Thanks in advance.