We installed a brake booster into the Zombichero, it worked out pretty well, minor firewall mods, used a Fox master cylinder, only had to fabricate one brake line, however...
Driving the vehicle has decent brake feel and stops much better except, the pedal is very low and the lower the rpm the less pedal it has.
The vacuum line was plumbed to the carb, is this the mistake we made, should it go to the intake manifold?
The booster was a NOS unit, it seems to be fine but could it be losing vacuum?
Thanks in anticipation
"Ranchero BTW, dammit"
should be a check valve in the line between the booster and the intake manifold, so when you open the throttle you don't deplete the vacuum in the booster.
Is the right angle plastic piece off the front of the booster the check valve, I searched around and found various pics with and without inline type check valve.
Yep, intake w/check valve. Pull off the elbow and see if it's hollow.
Moved the vacuum line to the intake manifold, replaced the check valve, (old one was a valve but we had doubts it was good) start the car and the same problem.
Pedal is low, definitely shows boost and assistance but so low to the ground it hits the floor when you hold it down.
Started the car, foot on pedal and as it build rpm in neutral the pedal sinks, is the booster junk? Is the rod from the master to the booster too short?
Any ideas welcome
are you saying it's a NOS ranchero booster to which you've bolted a Fox master? If so, it's possible that the booster output rod isn't long enough to properly engage the MC piston.
engine not running, foot on brake, start engine, pedal drops = sounds pretty normal to me.
Build rpm in neutral and the brake pedal drops further.
Start the vehicle and let it run to build up vacuum. Put your foot on the pedal, and shut the engine off. Does it push your foot back up? If so, you have a vacuum leak. Some boosters require an air tight seal between the master and the booster. I have no idea if yours does. If its a leak at the pushrod, it should be audible.
Its also possible you have the wrong pedal ratio, or your shoes need to be adjusted. The naturally long stroke of the manual brakes may have made it seem normal, and now it just feels weird.
I vote pedal bore or M/C ratio is wrong. If the Zombiechero had manual brakes before then it's pretty much got to be the pedal ratio. Typically a power brake pedal ratio will be somewhere around 4:5-1 and a manual will be around 6:1.
i play with GM's, but most of them put the vacuum hose for the booster right to the back of the carb..
i'd also bet it has something to do with the pedal ratio: they don't put brake boosters in at that goofy angle to make them look cool, they do it to give the pushrod a straight shot to a point on the pedal that's down closer to where you push with your foot.
also, it's possible that you have the wrong pushrod between the booster and master cylinder, and that the master cylinder itself is the wrong bore... the lines might hook up to the master cylinder different, too- the line from the big reservoir always goes to the front brakes, regardless of which end of the master cylinder it's on. hook them up backwards and the pedal will feel totally wrong and the car won't stop for crap.
regarding the check valve: the fitting where the hose hooks to the booster is usually the check valve, sometimes there is a thing that looks like a fuel filter inline that is there to keep the gasoline fumes from getting into the booster and eating up the rubber diaphragm.
I know when I was looking to add power brakes to my '70 mustang, a different pedal / pushrod were needed if using OEM booster and MC, or the leverage ratio would be wrong. There other solutions to add power brakes with the existing pedal, but that was pretty much buying a complete kit for $$$. This in the very early days of the internet, and well documented DIY solutions (other than all OEM) were hard to come by.
Check and see if there were two different pedals for power & manual brakes.
Piggy-backing off what Andy said, a little while back I converted a customer's '69 Mustang to power brakes. The "kit" wasn't a kit as advertised, just a booster and MC.
Bolted in with the stock manual brake pedal the symptom was similar to yours. Pedal was almost to the floor, maybe 1" of travel at best. Skipping over all my bitching about the conversion kit company, modifying the pedal ratio by dropping the pushrod attachment point 1" on the original brake pedal fixed it.
I know on the Mustang Ford used two different brake pedal assemblies, with the ratio just being one difference. The "kit" introduced a 3rd pedal that they sell (but not included in the kit) that was manual pedal with the pushrod attachment lowered.
Messed around making a couple of spacers, placed them between the rod and the master cylinder and it worked, the pedal was immediately better, brakes feel awesome for a 1972 vehicle and full confidence in the system was restored.
Thanks all
piling onto the concern expressed by iadr, is thsi "spacer" completely secure to the pushrod such that it can never never never become dislodged and cause (1) the pedal travel to go to E36 M3 or (2) bind/jam/otherwise block the system from building pressure?
