Really interested in this , Carl …Yes, if master rod, (nothing to do with power boost yet) ,is going that far into master cylinder, such that pedal is near floor, power or not , you are displacing that much brake fluid,(a lot) so either air in system or you are actually moving the disc piston in and out that far . (this by way of helping diagnose this ) Check also that back brakes are adjusted perfectly. ..look under dash and check, but I believe that pedal rod motion has to be following pedal, not adjusted or something. . .Gap of master rod at cylinder piston must be there too, with pedal all the way up, so inner part of master comes all the way out, an internal stop there, in master . pedal must be not depressing it at all with pedal up—that lets master refill with new or added fluid from reservoir if needed . ; slight gap ( hate to give a number due to interaction with boost, factory adjustment , but guess about .050 minimum). But do not adjust it! if the slight free play is already there , you can feel the obvious slight gap. If wrong adjustment can lead to lock up too. I believe you do need always a low pressure check valve to keep disc pads out , almost resting on the disc. I believe it is 5 psi? 10 on drums if need be on drums (varies) stock drums had none , but anti return low psi valve keeps brakes out, so master needs to move less fluid to activate the brakes next time (= less pedal travel) . See wilwood site on right psi. .
A larger bore moves more fluid, yes, but also lower pressure in lines, so less braking for a given press effort when all is working right . Reason for my interest , is getting manual brakes with disc to work, on F , requires small piston, other changes (pedal arm ratio) , nothing to do with this situation, but-------- what you are dealing with, on CC of fluid to work disc , and pedal travel with or without valve is very good information. I would add that 5 psi valve to front discs. (wilwood) Sometimes it is in certain masters already, but very confusing. The 67 master may have also had that (XXX) Mopar stock valve with a wire on frame , what it really does has been banged around a lot, (why XXX) but I believe mostly it shuts off either front or back lines if one bursts (not a proportioning valve!_) which lights a red brake light. Centering that so light is off can be a dog. There may be differing versions on this A,B E and truck body frame valve , but many other cars do not have it at all and not sure it has anything to do with keeping pads out, but often , “pressure valve “ or “proportioning” is attributed to it. Not on the one I got into. Maybe some a or e body have anti return at 5 psi for discs in the master. (Disc/drum from factory) And some kinds of discs may not need it..different brake suppliers across those years. And drum / drum common. But two valves in line should still be OK , doing same thing….important thing is to keep disc pads somewhat out?
Hope this info helps. Report back any info you learn?
I do have a car with both anti return valves, outside the master, manual brakes, Mustang disc / Ford drum (Studebaker with hemi) has corvette master which is a big bore. But light car; it works ok. Stude pedal ratio an unknown, so do not read a lot into that. .
I finally found some info on that frame valve in a 78 or 79 Dodge light truck book, which has a cross section of what is in it. metal bar slides front to back, closing off any opening with a high flow, or PSI difference front to back, is all I could see. With PSI the same on both ends it will not move, just sits there. Any sliding of the rod either way grounds the wire on it (notch in middle) . On the truck anyway. I only get into that as it can be a red herring confusing master cylinder use. . Lots of stories. . What looks like a pin to center it sticking out is not!!
As far as hard pedal with car vacuum off, that is just manual unboosted brakes you are feeling , but with a totally wrong pedal ratio, so takes two feet plus, to stop. Should not impact any of above except boost helps press more when vacuum is there..as it should. Is stopping action good, even with low pedal ?
Last if by chance silicon brake fluid, getting air out once in, is beyond belief hard. Never get air in ,in first place, is only real answer ,when pouring . never ever shake. Takes six months to sit before gone.
Let us know how you do and pedal travels?
Good luck,
John Grady
Hi all, I have done the AAJ conversion to disc brakes with Chrysler discs, Chevy Monte Carlo calipers and MC 67-70 E-body etc. and proportion valve as recommended on my G.
The brake pedal is perfect firm and high with engine off, but with engine on, and apply the pedal it almost are hitting the floor. The car stops, but I missing the last punch.
I have checked almost everything and is now suspecting that the for me unknown bore ( <1"?) of the MC is to small.
-76 Monte Carlo and have a single 3" piston (7.07 sq in. area) I could believe that this should require a larger bore then from what I have found out could be on my MC, on the other side I understand that this combo is used frequently without issues.
Could it be that I have a model of MC with a residual valve should? / Could it be removed on this MC?
Regards Carl
