This is bizarre. I have a Mack Brass euph that I've been using for the past 3 or 4 years, and have seen nothing like this. One question I'd have is: Have you CLEANED the instrument since you got it? I mean, flushed the whole thing out with soap and water. That's the first thing I do whenever I get an instrument. I can't imagine what the black gunk is. Did you buy this horn new from Tom McGrady at Mack Brass? If so, I'd email or call him and ask him what HE thinks. He's very responsive and stands beside his products. If you bought it used, then, alas, there's no telling what's been in it.
In terms of compression, the loud "pop" is not necessarily a valid test of adequate compression. However, on that horn, I do get a pop in each valve circuit.
Since I got the horn, I've been using light synthetic valve oil in it. Originally this was Alisyn. But this was actually a bit too slow, and I switched over to Yamaha Light valve oil. I'll oil the valves once very two or three WEEKS. If I had used "moderately thick" oil on my pistons when I got it (or now), they'd be too sluggish to use. They're TIGHT.
I like the horn so much that I've been about to email Tom with another "thank you" for making it available. I've been playing it a lot this year and in fact have a performance using it tomorrow. Tom has a 2-year warranty on all his instruments, and a total money back warranty on this particular horn. So my experience has been nothing like yours, and assuming that you bought the horn from Tom, you should get in touch with him right away. Even if you didn't buy the horn from him, you might get in touch with him to see what he thinks.
Last edited by ghmerrill; 12-20-2014 at 09:35 AM.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)