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Thread: M/C w/ Booster question ( 4 wheel Drum)

  1. #1
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    M/C w/ Booster question ( 4 wheel Drum)

    Hi guys! Ok, i have 4 wheel Drum brakes on my vehicle. My Master Cylinder was originally a Single circuit design designed for the 4 wheel drums (Manual brakes, no Booster). So now i put in a Dual circuit M/C which i believe might have been designed for front wheel disc brakes and rear drums. The m/c is situated at the top portion of the firewall above the brake assemblies.I havent experienced any problems so far (which is why i dont intend as yet to put any Residual check valve). I do plan to fit a Brake Booster but have some concerns in case i cant find one with the correct rod length for the pedal rod (the clevis end) and the correct pushrod length (the rod in front that contacts the m/c . If the pedal rod (clevis end of the booster) is too long inspite of clevis adjustment, i can always use spacers between the firewall and the booster or find a shorter clevis. But if these dont work, will it be okay to possibly cut a portion of the pedal rod end and just join them with an interference fit tube? Also, what about the pushrod end of the booster (contacts the m/c), if inspite of adjustments done to the booster pushrod its still too long,can i also just cut a portion of it and have an interference fit tube/sleeve fitted to it? And what about if the booster pushrod is too short, can i just cut something and attached it to the booster pushrod to lengthen it? It might be next to impossible to find pushrod extenders in my neck of the woods! Can these pushrods be safely pulled out to be modified if necessary?

    Another thing, is there any other quality of a booster that i need to asess, other than the ff:1- It is what fits the engine compartment. 2- The m/c fits it correctly. 3-Proper clearances between the m/c and pushrod, as well as proper pedal rod length-adjustment. 4- The m/c- booster generates satisfactory pedal effort for the strength of my legs. What about things like the strength of the springs and such inside the booster? Anything else?

    Any suggestions and tips you have would be appreciated as i plan to buy one in about 15 or so hours. Thank You! patrick
    Last edited by pat; 02-03-2012 at 09:45 AM. Reason: added stuff

  2. #2
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    Brake system design is not something that you want to "cobble together". I would look for a M/C-booster combination that was designed to work together and that will have the appropriate bore diameter for the rest of your system. You will probably need a proportioning valve to be able to set the brake balance. If the master cylinder is designed for drum brakes it will have residual pressure valves built into it to keep some pressure in the lines to hold the cups in the wheel cylinders against the cylinder walls to prevent leaks. This also puts some pressure in the lines so you don't have to use too much pedal travel to activate the brakes.

  3. #3
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    Patrick,

    I can tell you from experience that you will spend far too much time trying to mix and match parts. While more expensive, it is much easier to buy a complete package. The problem is that the master cylinders for non-boosted applications have a deep pocket so that the rod cannot fall out when the pedal is released. The boosted master cylinders merely have a depression since the rod is attached to the booster and cannot fall out.

    To directly answer your question, yes, you can cobble things together and get them to work. Bu8t expect to waste a lot of time doing so.

    BTW, I am shocked your brakes work with a disc/drum master cylinder pushing a drum brake system. If all the parts are new, that might explain it. But at some point, as the brake shoes wear, your rear brakes will end up coming on much sooner than your front brakes and will probably lock up before the front brakes can apply full pressure.

    Jack

  4. #4
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    Sorry for the very late reply guys, been busy and the computer`s been acting up. Ok, in retrospect, i just assumed the mc i had was for a disc-drum combo, i might be wrong though(?) Its one of those aluminum dual circuit m/c`s (15/16") with the hole depth about 18.5 mm for some toyota vehicle ( i will have to find out what model to help me determine for what brake combo it is for). I decided to have a mechanic check it out and as it turns out the m/c i had was bad and so got a new m/c with the same specs. The mechanic adjusted drum brake lining clearances as well as for pedal linkage freeplay and claimed to have done things ok...until i test drive it and find out that my brake booster was dead (so all along he meant it was ok in the sense that clearances were good, so i got moderately high brakes w/out brake drag, but it was hard as heck of course!) I suspect the mechanic did the adjustment with an already dead booster, as a few times in the past he mentioned the booster was too strong for the m/c (is that right?), and that a none boosted application would get things right e.g. no brake drag, high pedal. He then asked me to observe how low the brake pedal went down (foot off the pedal) upon starting the engine (maybe only went down an inch), he says it was too low (is he pulling my leg?). Btw, i get about 20 inches vacuum from my vacuum source.

    I then did some vacuum tests on my booster (i just found out that it is actually a clutch booster for some isuzu vehicle) and finally opened it up and replaced a single seal, did tests off and on the vehicle and drove my vehicle (I did measure the pedal linkage clearance going to the booster prior to taking off the booster and fixing it). The booster worked well, except my brakes dragged pretty bad

    So here`s my question, since brakes were working fine (fine like for non-boosted brakes) with the dead booster, is it then just a matter of adjusting the booster pushrod to m/c clearance? Maybe adjust my pedal freeplay clearance as well? I read somewhere (think it was a lexus training manual or something) that it was best to do booster pushrod and pedal freeplay adjustments with engine on as the vacuum tended to somewhat change the pushrod position due to the flexible rubber diaphragm?

    Btw, is there any standard range of depth of the rear hole for the m/c that tells you if its for a boosted or non-boosted application?

    Guys, i know you warned me about experimentation. I`m not any kind of engineer, not even a mechanic, its just that i really am on a limited budget... Also, parts over here are limited to a very narrow market (mostly asian import vehicles). Perhaps you can recommend other sites as well, i do have a (sorta) decent grasp of highschool science subjects if needed. Your inputs are appreciated. Thanks Again!
    patrick

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  6. #6
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    Thanks for the link grant! I`d actually gone through it in the past, but just now read it more thoroughly, lots of good stuff.

    My brake drag problems are now hopefully solved! I decided to bring it to a technician a couple of days ago who simply adjusted drum lining clearances on one wheel that i observed tended to sometimes bind itself, apparently lining was way too close to the drum. Been doing well since then. Hopefully it stays that way... Thank You All!

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