55.2-59 Brake booster opinions

[tbrown5079]

I built a sweet 56 cameo with a 4.8 LS engine and a GM A body front clip. Lots of other upgrades. Happy with everything but the brakes. I used a firewall mounted kit with a 7" power booster and swing pedal bracket for under the dash from a popular Chevy truck supplier. I had the booster replaced once (lifetime warranty). The engine is stock with low miles and has 20" of vacuum at idle. The brakes work OK, but I just can't really set it down like I may have to sometime. They have 8 and 9 inch boosters with single and dual diaphragm that would fit my kit. I have room for a larger booster. The mounting is 4 bolt 3 3/8". Is bigger better and why do they use dual diaphragm now instead of single like they did before? Your thoughts? Suggestions? Tbrown

[dsraven]

they use dual diaphragm boosters to try to replicate the assist from a larger diameter booster. it all relies on vacuum applied to the surface of the diaphragm. smaller booster = smaller diaphragm = less surface area of the diaphragm so they install 2 diaphragms for more surface area. a larger booster would likely give you better performance but thats my personal opinion. either they are bigger around or they are longer and stick out into the engine bay further. if you are running disc/disc you may benefit from a dual diaphragm booster of a larger diameter single diaphragm booster. some use the hydro boost for that reason. small package and good brake boost assist. if running power steering you can use the same pump. they get a little "busy" under the hood due to the extra hydraulic hoses but they work great. guys that mount under the floor all say they work awesome for that application because they are a small package but give great assist.

https://techtalk.mpbrakes.com/single...hragm-boosters

[Black_Sheep]

When I finished my build the biggest disappointment was the pedal effort required for braking. Everything checked out ok but the brake pedal feel just didn't seem right. Last fall I replaced all of the hydraulic components and upgraded from a single 7" diaphragm booster to a dual 8" diaphragm booster, the result was a much improved "feel".

[tbrown5079]

Thanks to both of you for your opinions. I am hoping to solve this by buying the best system for my application and only change what is necessary. Not that money is no object, but I want to have it be right and move on to something else. Thanks again. Tbrown

[dsraven]

I would check the master cylinder bore size you have, which also factors in, also the mechanical advantage given by the pedal leverage (like how far away from the pedal pivot is the pushrod for the master cylinder/booster connection) and then choose a booster for your system, be that disc disc, disc drum or drum drum. Sometimes I compare our trucks to other trucks the same size roughly. A regular full size truck is bigger and has bigger brakes etc. An S10 is more like it really for size of brake components you have and size/weight of the trucks. They used G body brakes so smaller than a regular sized truck. A quick search I found they came with disc front and drum rear or disc/disc on some models. Disc/disc also uses a different master cyl because there would be no built in residual valve for the rear drums. They used a 9" dual diaphragm booster or an 11" single diaphragm booster. How does that compare with your set up?
A larger master cylinder means more pedal effort is required for the same amount of hydraulic pressure in the system, if all other things in the system stayed the same. this can be offset by a different pedal ratio, meaning a longer dimension from the pedal pivot to the pushrod or a longer pedal lever. Bear in mind that a longerlever also means more pedal travel for the same amount of fluid output at the master cyl. You can see that one thing affects the other so compare apples to apples. Using an s10 booster and master would work but to keep with the apples to apples theme you would also need to keep the pedal ratio the same, follow?

[LEEVON]

x2 on the master cylinder size. Check to see what size bore you have and size it down. If you have 1 1/8" bore, go to 7/8" and you will see a major improvement. I did the same on my '50 with 7" booster.

[jweb]

Here's some more good info on boosters
https://techtalk.mpbrakes.com/booste...ur-classic-car

[mr48chev]

When I subframed my 48 I ran a big ugly master cylinder and booster off a donor car that we scrapped out and that one would put you though the windshield with the same brake rotor and probably rear drum setup you have but it was horrendously ugly under the hood. That it dominated the whole under hood area and hid the engine from the drivers side is the main reason I don't like firewall mount boosters in old trucks even thought they usually work fantastic except when you get the wrong combination.

One thing on old trucks brakes or handling is your perception of what good brakes are from driving your daily driver. If you are driving a late model somewhat performance car or even half ton truck with 4 wheel disk brakes you may be expecting the 56 to stop the same way that one does and while it is actually stopping exactly as it should it isn't stopping like the daily. If you daily is a mid 70's C_0 the perception is going to be different.
Have you got a buddy with a similar truck with close to the same setup on it that you can have drive it and get an alternate opinion?

The other thing is brake pads and brake lining, Some just flat doesn't stop as good as others but may be quiet and not make as much brake dust. I've got a set of pads on one of my OT cars that I am not real happy with as they don't stop as good as the worn out pads I took off. I should have spent a few bucks more for a better performing pad.

[dsraven]

yup, good info. it's what I was alluding to as far as checking the overall needs before going further. sometimes better to start over and sell off the stuff you have than try to make something work that may be more trouble and not work as well in the end.

things you need to know before buying/changing anything
-caliper volume/wheel cylinder volume. large piston calipers or wheel cylinders need more fluid displacement from the master cylinder than smaller piston calipers/wheel cylinders would, so a larger bore master seems like the thing to use. some guys just think bigger is better. these larger bore masters, however, will require more pedal effort to achieve the same system pressure than a smaller bore master cylinder would, given the same set up scenario. the smaller bore master would require a longer pedal travel to displace the same amount of fluid but once the system pressure builds the smaller bore master could apply more pressure with the same pedal effort. make sense? if you install a larger bore master cyl and you decide to go out and do some testing but find the pedal is rock hard and you are pushing for all youre worth but the car won't skid the tires (no antilock system) then it is likely that the master's bore size is too large for your system. the pedal is hard to push because the effort required to move more fluid with that bore size is more than the mechanical advantage is designed to allow with that amount of pedal effort.
https://www.onallcylinders.com/2013/...fects-braking/
be aware that some of the calipers used back in the day (possibly the A body cars used as your front clip donor) had larger cross section of piston sealing O ring (square, not round like a normal o ring) so the pistons were drawn back into the calipers further when the brakes released. this was done to reduce brake drag but requires a master cyl that can displace more fluid, a quick take up master cylinder. if you don't understand that scenario let me explain. when the brakes are applied the caliper piston doesn't slide through the sealing o ring like some would think, it actually rolls the (square cross section) o ring in it's groove in the caliper as the piston moves the required amount. then, when the brakes are released, the (Square cross section) o ring wants to regain it's shape so it rolls back to it's original spot and brings the caliper piston along for the ride. this allows the brake rotor to spin freely without the drag that would happen if the piston didn't retract as far when the brakes were released. so, back in the day, they used a larger square o ring so the piston retracted further into the caliper. presto-boomo. less brake drag. that meant the master cylinder had to displace more fluid though. to get past this they came up with the stepped bore master cylinders. these displaced more volume at first using the larger bore piston, until a predetermined pressure was built up in the system, then the smaller bore piston would take over and allow better ability to build system pressure. if doing a comparison with other production vehicles roughly the same size/weight as your truck, like the chevy S10 for example, you will see that the s10's came with a step bore master cylinder.
https://www.jalopyjournal.com/forum/...ments/4532182/

https://www.freeasestudyguides.com/b...-cylinder.html

http://faculty.ccbcmd.edu/~smacadof/...B_a5_m03_Final

http://www.idmsvcs.com/2vmod/brakesy...es_lowdrag.pdf

https://www.aa1car.com/library/brake...r_cylinder.htm

-disc/disc or disc/drum to determine master cyl type. a disc front and drum rear will need a residual valve on the drum brake side and these are typically built into the master cylinder outlet but they could also be an added inline valve. sometimes a rebuilt master cylinder sold for disc/disc set ups still has the res valve in the outlet and sometimes a master sold for disc drum doesn't have the res valve built in,this can really mess a guy up if unaware
https://www.freeasestudyguides.com/r...eck-valve.html

-space available under hood to determine booster size requirements (my advice-go big or go home, if you have the space. lol)

-pedal type and mechanical advantage (dimension from the pedal pivot to the pushrod for the master cylinder) this also factors into the pedal travel thing

-pedal stroke available compared to full stroke needed on the booster/master cylinder. basically ensure you have the ability to do a full complete stroke of the master cylinder with all the carpet and floor mats (etc) thickness taken into account. remember a long pedal that is required to swing back and up, past a normal vertical position, to it's "top of pedal" spot will also be applying some downward application force because of the weight of the pedal. these may require a return spring and a pedal stop of some sort so the pedal always stops at the same place at the top end of it's travel. this is because the booster and master need some free play to allow the master to fully retract no matter what. otherwise when the brakes heat up and the fluid tries to get back into the master's reservoir it can't because the brakes are slightly applied from the pedal weight and the return holes are covered. this results in a slight brake application. sometimes enough so the wheels don't want to rotate until the system cools off.

here is a bit more info to muddy the waters. hopefully that didn't shut your brain off, lol.

https://www.joesracing.com/master-cylinder-math/

[dsraven]

like mr48 says, know the pads you are installing as well. some manufacturers advise to burnish the new linings. this usually means making a bunch of concurrent stops to heat the brakes up and cure the resins in the friction material. this makes the brakes perform like they should without the squealing from pads that got glazed up etc.

https://www.freeasestudyguides.com/b...ng-brakes.html

https://www.brakeandfrontend.com/tec...pads-or-shoes/

[mick53]

I have Wilwood 12.88" disk. 6 piston front and 4 piston rear with the drum parking brake. When I called them they set me up with an engineer and we worked out what it was going in and how I was going to use it right down to the weight of the truck. They then put together a custom package from the master cylinder to the pads. I was lucky enough to get the Nickle plated calipers for the short period of time they offered them. I was having my Frankland quick change built at the time and they called Frankland to get the width and flange right for their brakes. I like all my parts in a system to be the same supplier if possible. After all that they said if you buy it from summit they will be less money than from us. I asked them why and they said "they sell a lot of brakes". It saved me about $650. I have a 53 3100.

[tbrown5079]

Thanks for all the good advice. To be more specific, I purchased a nice kit that was perfect for what I was building. It has the under dash bracket, pedal, 8" booster, master cylinder with proportioning valve all together, It is disc drum. The rear is out of a 71 Chevelle. I always have brakes and the pedal is never spongy. It seems like a vacuum issue, but I have 20" of vacuum at idle. When I first start it, the pedal drops as it should. But when I start driving I have no power brakes. As I drive and use the brakes the booster starts to work. I have already sent the booster back twice. The first time the reservoir was clamped crooked and the second time the booster was bad. I just want to try changing things out one at a time to logically diagnose the problem. Today I got a 9" booster from an S10 so I will try that tomorrow. Once I figure it out I can get on the supplier to get me what I need. Thanks for your help. Tbrown

[jweb]

It sounds like you could have a problem with either the check valve on the booster or the push rod length.

Check Valve - Remove the valve from the booster and blow through it. Air should only go one direction. If air goes both directions or doesn't go through at all, the valve is bad.

Push rod - Here is some good info about adjusting the push rod:
https://techtalk.mpbrakes.com/how-to...r-cylinder-gap

[tbrown5079]

Installed the booster from the S10 this morning and can't believe how it just bolted up with no problems. The push rod is not adjustable so the pedal sits an inch or so higher than I would like but will work until I get a new one. This is an old booster off the shelf, and I have the best brakes ever. Now I can go back to the supplier with a strong case for them to make it right. Sometimes you have to jump through the hoops. Thanks for all your help. On to the next project. Tbrown