Seems like a good move.
The PA 38 Tomahawk is a light (1680 MTOW) two place powered by a sturdy and practically indestructible Lycoming 4 cylinder O-235. TBO is 2400 hours.
According to Vref, a well-known pricing reference, the average PA 38 flies 151 hours a year from the date of manufacture.
That means an average 1978 Tomahawk has over 6000 airframe hours by now.
Vref says average retail--mid time engine and average avionics--is selling from $24K for a 1978 one and $28K for a 1982 one.
Looking at some of the online sales sites, there's rarely a PA 38 listed that falls into that window.
Things that increase the value and modern avionics, ADS-B Out equipment, no major damage history, and a low time engine that was rebuilt by Lycoming or a reputable rebuilder.
A lot of airplane prices are going up but Vref doesn't reflect that for the PA 38.
I urge you to spend some time looking over the Piper Flyer website; there you'll find many articles with good advice on buying an airplane, obtaining valuable records from the FAA and getting a good pre purchase inspection.
A piece of advice; unless a good friend has one that you know is in good shape, be patient. The right one will show up.
The "pressure" is actually the vacuum in the plumbing hoses, vacuum regulator, artificial horizon instrument and the direction gyro instrument.
In the past the turn and bank instrument was also driven by the vacuum system but those have been replaced by electrically driven T & B or turn coordinator instruments.
The first thing I would do is replace the central vacuum filter. Somethings they look clean but aren't. This can cause a restriction to the free flow of air into the system.
Do you know how long the vacuum pump has been installed in your airplane?
Do you know if you have a wet vacuum pump? or a dry vacuum pump?
I'm going to guess your pump is a dry pump.
The failure mode of a dry pump is immediate; so many pilots that fly instruments change their vacuum pumps every 500 hours as a hedge against a sudden pump failure.
Some shops have a portable vacuum system test kit. If a local shop does, it can test for leakage in the system that would give you a low reading.
Some vacuum pumps have a "vane wear" port which permits shops to accurately determine how worn the vanes in the pump are; if they're worn below a certain level, the pump should be changed.
I have heard of shops that adjust the vacuum relief valve to "bring up" the vacuum gauge reading, which should be between 4.8 and 5.2 inHg. This, in my opinion is counter productive; it's putting a bandaid on the problem.
Change the central vacuum filter (type "vacuum filter" into the search window on www.aircraftspruce.com
for an idea of what I'm referring to) and let me know if that makes a difference.
I might not give you a correct answer without knowing more about what you're doing.
Generally, though most rivets used on Piper aircraft are designated MS20470-x-x for a universal head rivet or MS20426-x-x for a countersunk rivet.
These rivets have a small dimple in the head of the rivet.
Some vendors use AN 470 and AN 426.
The first number in the dash series is the diameter of the rivet in 32s of an inch. The second number in the dash series is the length in 16s of an inch.
A MS20470-4-8 would be a universal head rivet with a shank that is 1/8 inch in diameter and 1/2 inch long.
Please describe your needs so I can better help and explain the system.