Adjusting Brake Regulator Options

[bbrock]

I just finished rebuilding my brake regulator with the PMB DIY rebuild kit and it looks like new. Only thing left is to set the switch to 525 psi. I'm guessing I could send it to PMB to have them set it, but wondering if this could be done locally at a brake shop. Anyone know?

[Superhawk996]

Any pic of the internals and the valve assembly in the manual or from when you rebuilt yours?

I've been curious about whats in the "box" and thought about disassembling mine before sending. That lasted about a minute before I concluded I need to focus on the more important structural repairs. (IMG:style_emoticons/default/welder.gif)

Most proportioning valves in old school cars and trucks (i.e. pre-ABS) are set up such that they build a pressure at a fixed rate (gradient) in proportion to the fronts and then they level out at a set pressure and then flat line at that pressure. In a sedan this is usually set so the rear brakes don't lock up in a light load condition once weight is transferred to the front axle under braking. There are other versions with mechanical linkages to try to vary that proportioning relative to load and/or body position. Example I have a MotoGuzzi that changes rear proportioning when a rider or heavy load is on back of the bike. Many trucks ran setups like this to account for the big swing between driver only and max Gross Vehicle Weight (GVW) with payload and trailers.

I assume the 914 valve is doing something similar but since the vehicle doesn't have a big weight delta between light load (i.e. driver only) and GVW at 2 passengers + luggage it is probably less "conservative" in that gradient is steeper and the 525 psi max limit is very plausible to keep from locking the rears.

However, back to your point most proportioning valves really aren't a rebuildable affair on any modern (say late 70's to mid 90's) vehicles. I certainly haven't seen everything that is out there but in my experience the 914 valve is a bit unique in that its adjustable outside of the plant. Most that I've seen in mass production are just a check valve set via spring forces and orifice size at the time of manufacture.

[bbrock]

Any pic of the internals and the valve assembly in the manual or from when you rebuilt yours?

I've been curious about whats in the "box" and thought about disassembling mine before sending. That lasted about a minute before I concluded I need to focus on the more important structural repairs. (IMG:style_emoticons/default/welder.gif)

Most proportioning valves in old school cars and trucks (i.e. pre-ABS) are set up such that they build a pressure at a fixed rate (gradient) in proportion to the fronts and then they level out at a set pressure and then flat line at that pressure. In a sedan this is usually set so the rear brakes don't lock up in a light load condition once weight is transferred to the front axle under braking. There are other versions with mechanical linkages to try to vary that proportioning relative to load and/or body position. Example I have a MotoGuzzi that changes rear proportioning when a rider or heavy load is on back of the bike. Many trucks ran setups like this to account for the big swing between driver only and max Gross Vehicle Weight (GVW) with payload and trailers.

I assume the 914 valve is doing something similar but since the vehicle doesn't have a big weight delta between light load (i.e. driver only) and GVW at 2 passengers + luggage it is probably less "conservative" in that gradient is steeper and the 525 psi max limit is very plausible to keep from locking the rears.

However, back to your point most proportioning valves really aren't a rebuildable affair on any modern (say late 70's to mid 90's) vehicles. I certainly haven't seen everything that is out there but in my experience the 914 valve is a bit unique in that its adjustable outside of the plant. Most that I've seen in mass production are just a check valve set via spring forces and orifice size at the time of manufacture.

These are from this thread on the Pelican site showing the guts of the thing and are better than the pics I took.

(IMG:http://www.914world.com/bbs2/uploads_offsite/forums.pelicanparts.com-20845-1549589845.1.jpg) (IMG:http://www.914world.com/bbs2/uploads_offsite/forums.pelicanparts.com-20845-1549589846.2.jpg)

You are correct that it is just a spring-loaded check valve, but the spring force adjustment is external. PM me your email and I'll send you the relevant parts of the factory manual. They have a pretty good diagram and description of operation. I don't know if this was common for these devices of the era, but valve can oscillate on and off when the pressure exceeds the regulating force to pulse pressure to the rear brakes kinda sorta like an abs. I thought that was cool.

The "rebuild" just consists of cleaning everything up and replacing seals. Two o-rings, one on the valve body and one to seal the cap nut, a wiper seal in the valve body, and a gasket between the valve and spring housings. Nothing else to do in there. Setting the pressure on the large spring is the only adjustment. This is interesting though. The manual gives different changeover pressures where regulation begins for the /6 vs /4 which makes me think the valves are different.