Brass Push>Connect Air Brake Tube Fitting, 0.25 Inch Tube OD Union Bulkhead

150 psi is actually right at the rated working pressure for this fitting, so you're not pushing anything. It holds from full vacuum all the way up to that 150 psi ceiling, which covers standard air brake system operating range without issue. The Ethylene Propylene O-ring is what does the sealing work, and it's built to maintain that seal across the full pressure range.