Community Forums

**CountryFit** · 12-19-2015, 04:02 PM

my engine is isc 350. i have an old pacbrake exhaust brake. it works but it leaks air from the contact between the brake and the turbo. spending much time trying to fix it, well, to no avail, found out the cause was brake contact being warped - no matter what, it will not touch the entire turbo contact tight, even with a gasket.

options to me -
1) take the brake assembly out and find a machine shop to re-surface the contact. don't think it's going to be cheap.
2) get a new pacbrake. i saw a new PRXB on ebay selling for $670, but it says no "cobrahead". do i need a cobrahead to make it fit? i asked the seller she made no clear.

i heard PRXB is real good. is it a plug and play? what about the cobrahead thing? expecting educational inputs.

palehorse89 · 12-19-2015, 04:13 PM

From what I have read on here over the years, people that have installed the newer one, stated the difference was not worth the money. If you have to buy one, why not go with the PRXB model. As far as the cobrahead thing??? never heard of this. There is a mounting system call the "Full Marmon Mount" and the "Half Marmon Mount" for the exhaust brake. Do a internet search to read up on the mounts/flanges. My EB has a full marmon mount on my ISC.
https://qsol2.cummins.com/info/qsol/...nondodge.print

Look at the diagrams, the "Cobrahead" must be the cast adapter? guessing. My setup is figure #1 ,no adapter.

**CountryFit** · 12-19-2015, 04:51 PM

thanks for the fast response. from your attached doc, fig 1 is how my exh brake is connected. now sure which part is the cobrahead adapter (could what "cast adapter" in the chart be?)

Dutch Star Don · 12-19-2015, 10:48 PM

"Country Fit".....PacBrake makes the PRXB to fit several configurations. There are several inlet and outlet sizes that need to match what you have. I had one someone gave me for free, but it was such an odd inlet/outlet size that it only fit one model engine that was not very popular. I couldn't even give it away. I've never heard of a "cobrahead" fitting.

I needed to replace my exhaust brake on my 400 ISL and went from the standard PacBrake to the PRXB. I read a lot about it and knew that it was going to improve my braking, but not where I really wanted the inmprovement.

On my previous coach, I wanted more braking action at higher speeds, like 55 plus mph. The PRXB doesn't help at higher speeds, but will give you more/better braking at lower speeds.

The PRXB gives you better braking at higher RPM's, something you won't see at higher speeds. As soon as your engine starts to rev, the Allison will upshift and kill the advantage of the PRXB.

So......it will help, but at lower speeds. If you need to replace your exhaust brake with a new one, I would definitely spend the few extra dollars and get the PRXB vs the standard model.

hypoxia · 12-20-2015, 07:45 AM

I don't think you'll really notice a huge difference but if it works as advertised then it's worthwhile.

I'm puzzled about warping on your existing Pacbrake.

WHY UPGRADE TO PRXB?

An exhaust brake relies on exhaust backpressure to create braking horsepower. The problem in the past has been that as a motorhome slows down, the RPMs also come down and you lose backpressure, resulting in a loss of braking power. The PRXB addresses this very issue with the patented air valve (waste-gate) that is designed to close as the engine’s RPMs begin to drop. The closing of this valve creates higher backpressure and therefore higher braking horsepower.