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Brake booster Substitution

 
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Patrick  



Joined: 05 Dec 2007
Posts: 278
Location: Manila, PHILIPPINES

PostPosted: Sun May 25, 2008 4:28 pm    Post subject: Brake booster Substitution Reply with quote

This is my first How-To write up, as well as my first try at Brake Booster substitution, and as i am still unable to post pictures and am not writing every "little" procedure please proceed with caution with whatever i say.

Any comments are welcome. Originally i was going to post it in General Discussions just because i wanted to share my excitement over what i did ( excitement because i`ve been driving my 924 for about 10 yrs now with a NON-working brake booster).I never bought a new booster for economic reasons, not even a used one as i`d have to import one to over here, which means international shipping charges, yikes! But it might be helpful if i put it here instead. So here goes:

1)- I eyeballed the brake booster in my 1979 VW Passat to see if it had the same general measurements as the broken booster in my 924 which it did save for the booster rear Pedal pushrod which was 1.75 inches TOO LONG(more on that later).

2)- I disconnected the vaccum hose going to the booster and instead connected the hose from my friend`s hand vaccum pump w/gauge to the booster and pumped it up to about 20 inches of vaccum to check if the booster would still hold a vaccum. It eventually lost all the vaccum(0 inches) in about 21 minutes(but it was usable none the less), i believe due to a less than ideal airtight hose connection.Lacking a vaccum gauge you could always suck it yourself and "feel" the strength of the vaccum AT YOUR OWN RISK(old brake fluid, rust,etc..).

3)- So i took the booster off my old VW Passat and measured general dimensions, and mounting bolt holes for the Master Cylinder and booster mounting plate (CONSOLE #8 Fig. 9.2 in the Haynes manual). I also checked that there would be(VERY IMPORTANT) 1-3mm of CLEARANCE between the front tip of the booster pushrod and the master cylinder piston.

I checked this clearance by connecting the booster to the master cylinder and then found a way to push the pedal pushrod by hand at the rear of the booster until the Front booster pushrod made contact w/ the master cylinder piston(can be lightly felt at the pushing hand). I just eyeballed how far forward the pedal pushrod at the rear of the booster had moved and saw a definite 1-3mm freeplay clearance. DO NOT use any brake booster that does not allow this front booster pushrod to Master cylinder piston clearance or your brakes will drag and give you real headaches!

Btw i didnt have to disconnect any hard brake lines from the Master Cylinder as there was enough space in the engine compartment to CAREFULLY move the master cylinder out of the way somewhat so as to get the old booster out and to put in the VW Passat booster(AFTER i had a machine shop shorten the booster pedal pushrod and after i adjusted the clevis fork at approximately the same length as the original one).

Oh, about the SHORTENING of the booster pedal pushrod, after they machined off the right length , they joined it with a STRONG thick walled and Very TIGHT PRESS FIT Sleeve. I certainly entertained thoughts of cutting the booster pushrod witha a hacksaw and then slipping on it whatever tube i could find, then putting epoxy wherever i felt it needed, i decided against it...

4)- I then did fine adjusting(Another VERY IMPORTANT procedure) on the Clevis fork of the booster brake pedal push rod so as to have a Pedal Freeplay at the Brake Pedal of 8-10mm(although the HAYNES manual says 6-8mm), you measure this freeplay by lightly and slowly pressing the brake pedal by hand and when you feel a light resistance(this is when the front booster pushrod end contacts the master cylinder piston).

If there is no clearance(which means the distance from the rear of the booster to the mounting hole on the clevis fork is too long), then you risk dragging your brakes! When in doubt, i go for several mm. more brake pedal freeplay(shorter distance from rear of the booster to the monting hole on the clevis fork), of course way too much brake pedal freeplay will give you very low brakes, which you dont want because if for any reason you have a brakefluid leak in either brake circuit, the brake pedal may run out of space to fully apply the brake circuit that still works. I will likely still put steel epoxy at the point where the sleeve meets the pedal pushrod for extra strength. Oh, when removing and mounting the booster mounting plate(Console), it will help to have really small hands and/or one of those flexible thingys that connect the socket to the ratchet, this is where i spent a lot of time.

Of course it may be cheaper to just buy a used booster for you guys over there, but as i said earlier, not for me. I spent the equivalent of only about US$18 for the machining, and just a few bucks for vaccum hose and clamps and reused my old booster check valve(one way valve). ANY comments are welcome. I`m sure i missed a lot of points, this is after all my first try at Economy brake booster substitution. I hope you didnt get too dizzy reading this!
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