I've got a 3700lb truck that has manual brakes. It stops OK, under normal use. But if I had to make a sudden stop....well I dont know. Under hard ( panic stop mode) IT WILL NOT LOCK UP at any corner. Heres whats in my system 7/8 mamual master cyl (ford fairmount) pedal ratio 7.6/1 11" rotors/ 9.5" drums Caliper bore 2 15/16 GM combanation valve Pedal feels fine through out its range of movement. Not hitting any obstructions to limit movement. I increased pedal ratio to 7.6/1 it helped. I also put in the 7/8" master cylinder , it also helped..but still worry , that someday I might not be able to stop in time. Most of these parts are from a 77 chevy nova (except MC) All lines are in the propper location/ no air in system/ no firewall flex/MC on firewall. GM used most of these parts on 77 monte Carlo's that weighed 3968lb/ and yes you could get a 77 monte carlo with manual disc/drum brakes. I have plenty of VAC 22" , and could put on a booster. But would like to fix what I have. Any idea ???
I have a system that's almost the same. It works fine but my car weighs 2600lbs. I think a booster is your best bet.
If I would install a booster. Would a 7" or 8" single be enough? I would prefer not to use a double booster. And like everyone else, the smaller the better...but I do want this to stop better. What I can't figure out is..GM used almost the same setup in the 77 monte carlo ( with a 15/16MC (tried that) and I'm sure less pedal ratio) and the monte carlo weighed 3968lb. ???
You did all the right stuff, next step is a booster as others said. Make sure of the gap between the booster and master cylinder is correct. I use play doh to check the gap. If the booster does not help, I would suspect the MC piston is hitting bottom. Wilwood makes a 1.1" stroke 7/8" bore MC.
Normal line sizes have absolutely no effect on braking !! Normal 3-4 12-18 quality hoses also have no noticeable effect !! To OP part of your issues may be due to the quality of your brake lining material. Much of it sold today is harder than needed for long life with power brakes, and the m*** marketers life time warranty. I was around automotive/industrial friction materal for a good part of my working life so have a bit of experience with different lining materials and friction coefficients. Your MC bore etc. sounds good,but use a minimum of a 8" booster and a double would be preferred.
The 7/8" master and 7.6:1 pedal ratio is about the best you can do in most vehicles without running out of total master cylinder piston(s) travel. I would change the GM fixed combo valve to an adjustable prop valve to make sure you are getting the most out of the rear brakes. Factory fixed value combo valves were calibrated for a particular car/truck family and front/rear brake/engine/tire size/GVW/wheel base, and should really not be used on custom non-stock systems. (regardless who sells or promotes them) My experience with 7" single diaphragm boosters on mid size cars is not good, as they go into "run out" (manual mode) very early and are IMO a waste of money. I would at least use a single 8", but a small tandem 7" or 8" is a much better choice. You'll have to reduce your pedal ratio back to 3 or 4:1 with vac ***ist. Keep in mind that locking up the tires will result in worse stopping distance in a panic situation vs. keeping them out of lock. Measuring brake performance based just on lockup is not the best criteria. Ideally, you want the tires to just barely squeal with a panic stop, as this is the best brake performance. The tires will also last much longer.
i agree with John Evans, i have had similar problems and switched to a "cheaper" non lifetime type of pads/linings to get the brakes to feel right. is this a new system? are the drums turned? brakes "worn in"? shoes adjusted?
That Monte Carlo you're talking about probably had a 1" master. Just sayin, the factory did a pretty good job piecing it together. I like the 1" corvette type, worked great for my mismatch disc drum setup from 3 different cars. When I switched to a jag front, ( 4 piston calipers) I needed more fluid and had to go with a 1 1/8 boosted master and combination valve.
I'm not saying that sliding the tires will impove stopping (thats why ABS systems prevent sliding tires) ...my problem is safe stopping distance, and the fact that it cant even slide the tires in a panic stop.
The rotors are brand new, drums seem to be fine, shoes worn in. The disc pads are from AdvancAuto parts. Premium semi metalic. When you say you used cheaper pads, it seemed to help. How much did it seem to help you stopping distance/ sence of saftey ?
I've read that there are different friction codes on brake friction material. DD, FF etc. Which of these would make the better stopping difference ? I do not care about pad life. just want to stop in less distance.
The info about the combo vale makes good sence. Maybe I 'll be able to incrase the PSI to the rear brakes/ and get better stopping. Also as others have said, that maybe using non metallic pads should help. I'm going to try that also. I've read that there are codes on the edge of brake pads (FF,FF,GG. etc) does anyone know which code would provide the best bite/ better stopping distance ? Thanks to all for your help.
If you Google "brake lining friction codes" you'll get plenty of edge code info. The problem may be finding the friction level you want for your brakes. Not trying to get personal, but do you know what value of pedal force you can apply to the pedal? This could be part of the problem if there is a physical limit, and would necessitate power ***ist.
As far as brake linings go, ask for ambulance or highway patrol type shoes. They don't last long, however
I'm going to check the pressure after the combo valve front and rear. As for leg force pressure..no idea. You would have to figure driver angle/pedal angle/ personal leg strenght abilty ..I would guess. From what I've read non power disc require around 1000-1200lbs psi. rears around 800lbs psi from the master cylinder.
Brakes don't know if they're powered or not, but discs do require more pressure than drums. Typical city stop-and-go driving will usually require about 300-500 psi with front discs, less with drums. With your 7/8" master and low 7:6-1 pedal ratio, you would develope about 1266 psi with 100 lbs pedal effort, about what you would push in a panic stop, and normally enough to generate wheel slide in light-mid weight vehicles. Checking the system pressures should be helpfull. FYI, you can use a bath scale, valve spring checker or similar to measure pedal force.
Thanks for the info about bath scale ( would have never thought of that..duh) . My system is manual. With the scale in the truck (surprised it would clear everything with pressure on it) with panic stop leg pressure, I can push 120lbs. with moderate leg effort 50-60. Have not yet pressure check with a psi gauge. But have ordered a adjustable proportioning valve (should have next week). At this point I'm thinking that my combination valve may have not been working right.
I just did a pressure check AT THE COMBINTAION VALVE. ( had now way of checking at calipers/wheel cylinders. The pressure going TO the front calipers light 300-500, panic 1500psi. The pressure going TO the wheel cylinders light 200-450, panic 590psi I should be getting the adjustable proportionaing valve sometime next week. It would seem that I will be able to send more pressure to the rear brakes (the rear of the truck is heavier that most/ and I have Bigs & Littles also). Which should help improve mt braking issuse somewhat. Hope this helps...some ??
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