Vacuum for my 1995 Fiat RAM 2500 Commins Turbo Diesel

I own a high mileage (330,000) Dodge 2500 RAM turbo diesel, which has always had poor braking performance. I purchased it new at the end of 1994. The problem appears to be from insufficient vacuum availability. If I apply the break for a long duration, at lower engine idle, the pedal goes firm and the air conditioning louvers flip to what I believe are their low vacuum states. I added an engine break to aid in long down hills and a vacuum canister to extend vacuum duty for long braking spells. The expensive vacuum pump has been replaced and leaks in the vacuum lines have been sought and fixed. This appears to be a vacuum duty issue. Is there a high capacity electric vacuum pump that can be added which will cycle on as necessary and then turn off at 16 inches of vacuum, which will provide sufficient capacity for my diesel truck? My mechanic is aware of small 12 V vacuum pumps, which he says would probably not have sufficient capacity.

Very little CAPACITY is needed as long as there are no vacuum leaks. At 15 years and 330K miles, you need to go over the vacuum system from end to end… The first step would be to check your existing pumps performance. Can it pull 16 inches? The next step is to be sure none of the old vacuum hoses are collapsing under vacuum and limiting performance. You could install a vacuum gauge on the system so you could see and anticipate braking problems…

Thanks for your input. From the original date of purchase I have pursued a fix for this, including effort with the dealer, Dodge Corporate and a well respected diesel mechanic. I have contemplated adding an instrument to tell me when the vacuum condition is poor but I already know by the A/C and brake behavior. Static tests with a gauge connected have shown the the pump and vacuum system work to specification. The solution appears to be a supplemental pump. I am looking, now specifically for a pump to solve the problem, unless a new idea or diagnostic talent surfaces. I intend the keep the truck as the newer vehicles’ cab sizes have decreased and I am very tall.

Have you posted your question (and looked at other’ questions) here?

One of the small 12 V vacuum pumps working together with the existing pump should help things.

If you’re losing vacuum just holding steady brake pedal pressure makes me suspicious there’s a problem in the brake booster it’s had since new.