I put a dual 7" booster on my Merc a few weeks ago. The vacuum is right on the borderline about 16"-17". The brakes are still rock hard and you really have to mash'em to get it to stop. I can't lock them up. Would a vac. canister hurt me or help me? I'm really not happy with the power brakes right now and have considered tearing it out again and putting manual brakes in. I'm using a corvette style M/C like master power brakes sells with 11" discs up front and a camnaro rear end. Any suggestions?
16" to 17" at idle should be enough ...( hell....I had 8" on my 32 and they still worked...)... to test the booster ---------- with engine OFF press on pedal....START engine ...pedal should drop a little as vacuum takes over ..... (you'll FEEL it---try it on a STOCK car so 'ya see what I mean...) booster might be bad .....or leaking.... you ARE taking vacuum from MANIFOLD --right ?? ( and at least a 3/8' line) jersey Skip
We have a chopped 50 Merc in our area that put the 7" booster on it and has never been satisfied with it. Personally I think it's too small for the size of the car.
I don't know too much about power brakes, but i do know it is probably not a good idea to take the booster off and putting in manual brakes since the discs need a lot more line pressure than the drum brakes...i think it is like upwards of 750psi, probably more, for the discs and about 500psi, maybe less for drum brakes. It is pretty hard to get to get a lot of pressure from manual brakes. I don't think it would hurt to put a canister in, and like someone else said, it could be the booster is too small for the size of the car. Just a thought.
I think there was a post a few weeks ago on this topic too. I have this TCI set-up in my 51 Chevy, Ive added a vacuum canister and the pedal is still hard. I am pulling 18" at idle. I plan on crawling under the car this weekend to see if there is something else going wrong that I may have missed.
I am watching this thread with interest. I have the same 7" booster and 69 Vette master cyl. Rock hard pedal, car stops but I think it should be better. I was planning on going through the booster system this weekend. If that doesn't give me what I want I may move the hole on my pedal arm to change the mechanical advantage.
Wow, This must be a common problem. Cuz I got it too!!! Small booster on my chebby, never been happy! Pedal is hard. Mild street motor not sure on vac*** but shouldn't be bad. What is the deal? FONZI
I don't know what you guys mean by a hard pedal. I always thought a hard pedal was a good thing, as opposed to a soft, spongy, air-in-the-lines pedal. If you mean you have to push hard before it'll stop, then that's a different matter. If you have store bought brake kits, then the engineering is probabaly right but you could have a faulty booster, or you just might have put your residual pressure valves in backwards and they are making you exart that much more force before you overcome them. Or you have wussy legs and need to work out. I have a 7" booster that i boosted out of a wrecked '84 Mustang with a larger bore LTD M/C in my '4 GMC 10 bolt Malibu rear drums, ECI-Chevelle front discs, no proportioning valve. It stops fine and will lock them if a panick asks them too. I didn't think it was boosting much till one morning the engine died as I was approaching the end of my block and it took my whole leg to stop the sob, in fact I grabbed the ingnition key and restarted the engine as it was only slowing down and the restarted engine brought the booster back on line and it worked. Try an engine-off stop, (on a deserted street) to check to see if the booster is working. Or, if you don't have a "hard pedal" like you are claiming, but instead you have a spongy air-in-the-lines pedal, bleed the ****ers again.
The booster is brand new, barely a month old. I know the booster is good, I did "the test". Let me get into more detail about my pedal. I'm approcahing a light, I let of the g*** and place my foot on the pedal. As the pedal goes down about 1", the brakes start to grab, then it's like the booster just stops helping out. I have to put ALOT more pressure on the pedal to continue the stopping rate. Even when I try to panic stop, they absolutely refuse to lockup. I'm going to try to bleed them again this weekend and see what happens. DrJ, I've got a booster from a Ford fairmont wagon. Would it hurt anything to try to put that booster on the chev M/C?
7" booster is completly inadequate for a car over 2000 pounds, unless of course the system is designed properly. A 9" booster will give you more ***ist and may cure your problem, BUT! Most of you guys seem to be running the Corvette style master cylinder with a 1.125" bore. This is the hot rod industry standard and everyone accepts it for gospel, WRONG! if you inspect most OEM systems you will spy a smaller bore cylinder, somewhere around 1". If I am not mistaken the 1.125" bore cylinder only came on the Corvettes with the four wheel, four piston Delco Moraine calipers and they needed the extra volume. Your systems are much less demanding and the larger bore master cylinder only serves to increase pedal pressure. Simple hydraulic theory, http://auto.howstuffworks.com/brake.htm There are cases where you may have unknowingly installed the late model "quick take up" style GM metric (small body) caliper. If some of you remember I solved this problem for Cleatus some time last year. Symtoms of the quick take up type calipers manifests in excessive pedal travel before a firm pedal is felt. The larger bore of the 1.125" cylinder can mask the problem. In this instance it may be necessary to replace both front calipers with those from an earlier car, say 1978 Malibu/Monte Carlo and switch the master cylinder to the smaller 1" bore. First thing to try is the smaller bore cylinder. Remove your master cylinder from the power booster, measure the bore size with a caliper or small ruler. If yours is the larger type you should be able to pick one up a replacement from your local auto parts store. Measure the replacement to make sure you are getting the correct 1" bore. This should be from the late 70's Corvette. Otherwise you may check with the company you bought your system from and see if they will exchange the cylinder you have for the proper one. I know you will get some grief from them but you may have to stand your ground on this because you know the system you have is not working as advertised!
ElP, I know for a fact that the M/C is a 1.125" bore. Could you recomend a 1" bore M/C that you think might work in this situation? I don't have room under the floor of my car to fit any thing larger than the Fairmont booster (I've drug it a time or two pulling in and out of my buddies shop). My rotors on the MII front end are the larger 11"ers. I thought they were the Monte Carlo/Malibu rotors. My time is running really short to make the roundup! I don't know what else to do.
The 1 1/8 bore is definetely too big, plus the typical underfloor aftermarket pedal ***emblys don't have enough leverage. Get a 1" bore cylinder from a late 70s - early 80s Corvette. Make sure it has the shallow hole for the push rod or it will never work. It is just a dimpled hole where as the manual has about a 1" deep hole. Most street rod suppliers have those avaliable if you can't find one at a parts store. The leverage needs to be about 5:1 or 6:1 to get it to work good, but most of these pedals seem to be less than that. The bottom line though is these small boosters don't have much storage capacity, and will never be as good as a 9" or bigger booster.
I'm also forgetfull. You should also check to be sure if you have rubber hose running the full length that it isn't fuel line. It will **** flat and will do the same thing you are describing. You should run it most of the way with 3/8 steel tubing(with no kinks!) and connect it with vacuum hose at each end.
At 16" of vacuum your system is about 60-70 effecient. At least you have a "dual" 7" so the area is equal to a 10" booster. There are two things that make a hard pedal. 1. Wrong Pedal Ratio or 2. To big of a bore Master cylinder. The 1-1/8 master cylinder is NOT to big of a bore if you have enough vacuum to run it. The calipers you have on your car is ment be be used with a 1-1/8 Master cylinder and a 10 or 11" booster. I dont know how Master Power brake expects your brakes to work with the large single pistoon calipers. These OEM caliper require lots of volume and lots of pressure to operate and was engineered to be uses with a specific master cylinder and booster. When ever you change any part of what it was engineered for you have problems. i.e. little or no vacuum, different booster/mc combination, different size rotors, tires, distribution of weight on the vehicle, etc, ect.... I would bet that if you check your pedal ratio it is less the 5:1. http://www.hotrodheaven.com/store/brakes/coversheet.htm For information on master cylinder output pressure and calipers go to the above link. There is also a picture on how to figure your pedal ratio. DSO
[ It will **** flat and will do the same thing you are describing. You should run it most of the way with 3/8 steel tubing(with no kinks!) and connect it with vacuum hose at each end.[/QUOTE] i had that problem, im running the stock booster in my 54 merc, and i replaced all the hoses and stuff, and the hose that went to carb to booster got ****ed flat because of a kink so i had to buy a hose with an angle put into it already, and after i fixed that my brakes got ****ty again and i pressure tested my tank and found out that it had leakage at the neck of the tank, for some reason it was two piece, and it also leaked where the rivets where, so i welded and sodered it up and now it stops pretty good, but for some reason, right now it stops really hard, like i barely got to tap the brakes, and if i press on it kinda hard it will come to a screachin stop really hard, oh well its better then puttin all my weight and strength into it
moonmr is more than likely right about the pedal ratio. if you are pushing on a pedal with a ratio less than say, 7:1 you are gonna need an extra set of legs to get the leverage needed to stop the car. i have always read that a "stock" ratio was no less than 7.5:1, and no more than 9:1 (too high and your brakes will grab too hard to fast) figger your ratio, take the length of the pedal arm from center of pivot, to center of the pedel, and divide by length of the arm from pivot to pushrod. unless my math is way the hell off (likely), or i forgot the formula (more likely ), that'll give you your pedal ratio. anyone else? T
I guess you already made sure the inner pushrod ( the one between the booster and master ) was adjusted correctly. And the pedal rod is adjusted...sometimes you have to shorten or add to it.. Dont know if your kit has that but the stock older Fords do. If you have no boost it might be too short.
i had that problem, im running the stock booster in my 54 merc, and i replaced all the hoses and stuff, and the hose that went to carb to booster got ****ed flat because of a kink so i had to buy a hose with an angle put into it already, and after i fixed that my brakes got ****ty again and i pressure tested my tank and found out that it had leakage at the neck of the tank, for some reason it was two piece, and it also leaked where the rivets where, so i welded and sodered it up and now it stops pretty good, but for some reason, right now it stops really hard, like i barely got to tap the brakes, and if i press on it kinda hard it will come to a screachin stop really hard, oh well its better then puttin all my weight and strength into it[/QUOTE] If you move up to a larger bore master cylinder your brakes wont be so sensitive. You apparently have plenty of boost. DSO
A ratio of 7.5:1 might be to much and a 9:1 will never happen. Your master cylinders "normally" have a maxium stroke of about 1" to 1.2". So for ever inch of travel your pedal must move 7.5" on a 7.5:1 and 9" on a 9:1. Manual Pedal ratios run between 5.5 to 6.5:1 anything more than this is not physically possible in a normal car under "normal" driving conditions. Power Pedal ratios run between 4.5 to 5.5:1. For information on pedal ratios go to: http://www.hotrodheaven.com/store/brakes/coversheet.htm click on pedal ratio in the drop down box. DSO
Looked into my system this weekend, I have 16" of Vaccuum, I switched to a steel line to the booster, and I reduced the pedal ratio. Bled the system again an it's better but not good enough.. I checked my Master Cyl. and it is the 1.125" unit. I made the parts guy bring me 3 different Corvette master cylinders from 69-78 and they were all 1.125" Is there another application that uses the 1" bore master cyl? Is there a difference between a master cylinder for manual Vs Power brakes, if so what is it? TZ
I think all of the vette M/C's are 1.125. I know that the oreillys website will tell you M/C bores. I ran my vacuum line in 3/8 this weekend. It helped alot. I still have to stand on it to hold it at a light. I'm also having a rear drum drag. I'm sick of messing with it. I'm dropping it off at a friends shop after it comes out of the alignment shop.
I noticed my vaccuum drops way down at idle in gear. I have 16" at idle or cruising down the road, but idling at a light with the car in gear it goes to 5". I played around with putting the car in neutral as I a approach a light to keep the vaccuum up and it helps. Not a long term solution but it is part of my diagnosis. I'll check the O'Riley site. Can I ***ume the mounting face of the M/C's are all the same?
then consider a "vacuum reserve tank" ..the BIGGER the better !! also...put a check valve in line so vacuum can't be lost by backfeeding..... jersey Skip
May want to try the manual version. It will have a deeper hole in the end of the piston. I have made plugs out of 7/16 grade V bolts, rounded on one end and dished on the other to fill that void. You may want to try a Nova/Camaro cylinder. They may require a bit of modification to the lines but at this point if it solves the problem it would be worth it. If you want, give me a call and I can explain in further detail. Steve 602-278-6800
I'm ***uming this will be a 1" bore? (The next question is not meant to sound smart-***ed) What difference will that make? Am I correct to ***ume more line pressure? Am I heading in the right direction?
Here is my layman's understanding of this. Think of the size of the pistons at the master cylinder and at the wheels. The bigger the wheel pistons are than the master cylinder pistons, the more force you get at the wheel. So, in your case, since you are making the m/c piston smaller, the wheel cylinders are "larger" (relative to the m/c) , more force on the pads/or shoes at the wheel.
Yeah, simple hydraulics. I put up a link to How Stuff Works earlier in this post I think. If you followed through on all of the pages there was a working diagram giving a visual explanation. But yeah you have the idea going in the right direction. Smaller piston will produce a higher line pressure with the same effort at the pedal. You may have more travel in your pedal to displace the same ammount of fluid and that is the reason why the larger piston MC has become so popular. Little or no pedal travel! But as you discovered, very hard pedal. There is some trade offs that we have to deal with but it will be safe and reliable in the end.
The difference between a manual and power master cylinder is the depth of the hole for the push rod. A manual one has a hole about 1" deep and the power one is just a dimple. The parts guy should have an application that states 1" or 1 1/8, but if he doesn't, call a street rod supplier they all have them.
-------------------------------------------- You dont need a "Corvette" master cylinder. Any master cylinder with the proper spacing will work here are some part numbers for MANUAL brakes. 68-73 Chevelle Raybestos 36233 67-72 Mustang Raybestos 36251 these will have the back of the master cylinder bored for your pedal rod to fit into. dso
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