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After reading a couple threads about piston rod length and brakes sticking on i thought id check out mine.
my brakes work good, will lock up the tires and have a good maybe slightly firm pedal feel.
the issue is the front brakes seem to drag slightly after a little driving and get real hot after highway driving. Not enough to hold the car in nuetral on an incline but some drag when hot. I figured the piston rod was maybe to long. The booster and master cyl were newish looking when I got the car.
i seperated the mc from the booster and found no rod! Inside the mc i could remove a 1.25” inch long cylinder w 1 flat end and one end concave ( see pic). I can put my screw driver about 2” into the booster till it hits. I assume the part sticking out of the booster is piston rod retainer. The piece from the mc does not fit inside the protrusion from the booster. The protrusion from the booster appears to be to large to fit into the mc where the cylinder is, it hits the flange of the mc. It would appear to me this assembly has no piston rod, but needs one.
the adjustment at the brake pedal appears to be at the very end ( long as possible).
anybody have any thoughts or insight into this.
will post pics in a minute. Csm 1970 my cars a 1972 Protrusion from booster suspected to be piston rod retainer Cylinder in mc Adjustment at max
Last edited by RetroRanger; August 22nd, 2021 at 03:33 PM.
The adjustment on the pedal side is strictly for pedal height. As long as the part that pushes against the master cylinder is seating correctly and not applying any pressure at rest you should be good. If you have disc brakes then there will be a little bit of contact with the pads against the rotors and will generate a little bit of heat.
The "cylinder in the M/C" is a spacer plug used in aftermarket M/Cs. The M/C is drilled to accommodate the long pushrod on the 1966-older boosters. When used on later boosters, that plug lets the same M/C work with the short pushrod.
Your problem appears to be that the pushrod retainer is pressing on the piston in the M/C, which is why you still have brakes. Whoever did that install left the pushrod out and apparently overadjusted the pushrod from the booster to the pedal to compensate. Get a correct pushrod, readjust the pushrod to the pedal, and call it good.
Thanks for the replies. I looked on rockauto at mc’s and booster. I arrived at the same conclusion the last owner lost that rod and overadjusted to compensate.
i cant find a piston rod sold seperately. Are they availible? Anyone know the size? I would guess w the slug in the mc i could usea lonng or short version.
Joe, that's the first I've ever heard of an extension! I've bought a lot of repro boosters, and they used to include a long and short pin. The length is related to the M/C piston. Most M/C pistons are like flush. A 'vette has like a hollow piston, so you need a longer pin/rod. I learned this a long time ago before you could buy 2 bail repro cylinders for like my 69 442. I would buy 'vette cylinders. 'vette suppliers an others have them. I have some but I'm not in Winch now, not sure if I could even find them. You need to look at your M/C primary piston end. Search booster pin or rod(s). https://www.ebay.com/itm/15268562980...SABEgKbIfD_BwE
the slug in the mc looks like its there to accomadate the short or long piston rod boosters. On rock auto i saw mcs that specd flush or 1.5” depth hole. The slug makes it flush or if removed a 1.5” hole.
i didnt see any mc’s exactly like mine w the removable slug.
Sure Dean, well there was no slug used ever (68-72) to my knowledge. I think you need the long rod and no slug. I have no clue what that slug is from or where it came from. I suspect you have the wrong cylinder. I do see an adjustable rod on Summit. But it makes no sense to me because you can adjust the rod that goes through the firewall.
I dont fully understand its function, but its pretty common from my reading to question that piston rod and its length. Most post state the long rod is 4” ish and the short rod is 2 3/8 or 2.5”
so i grabbed a piece of 3/8” rod and put it in my drill and ground it to match the pic. I left it a little long and after several trials had it sized to leave abouta 1/16” space.
its raining now so ill check its functionality later this week
i found this picture on another site.
heres my tribute piece note the precision measuring tool
Last edited by RetroRanger; August 23rd, 2021 at 12:40 PM.
Sure Dean, well there was no slug used ever (68-72) to my knowledge. I think you need the long rod and no slug. I have no clue what that slug is from or where it came from. I suspect you have the wrong cylinder. I do see an adjustable rod on Summit. But it makes no sense to me because you can adjust the rod that goes through the firewall.
There was no slug used from the factory. It's an invention of the aftermarket. Again, this is to allow one M/C to fit both long and short pushrod boosters.
Right i agree the slug allows using a booster w a long rod or short rod , or i think in my case no rod, as the rod retainer appears different than the oe style shown in the manual pic above.
adding my rod made no difference, so i shimmed the master away from the booster. The brake feel changed to less firm but they still work fine and still drag more than typical, but not so much as to lock up,
further investigation also shows it appears to be only the fronts and ( shoulda saw this before) the brake light doesnt function.
i located the brake light switch wire wrapped around the harness at the pasthru. I plugged it in and you already know the light is on.
i removed the switch but cant tell anything from the hole other than there is a small orifice to accommodate the pin in the pin switch.
so what is the best way to recenter this. Apparently its been this way for quite some time.
am i right in thinking this is why my fronts are dragging more than typical?
Dean, I don't think shimming the cylinder is a good fix. Here's more info: there is a cup-like seal/gasket that goes between the cylinder and the booster. In your #1 post you can see it. It's almost always missing, they were unavailable except NOS for a long time, but now available repro. That could fix the shim issue or not. There is a tool to hold the prop valve centered, it's like $8.00 or so. Look on ebay for booster valve tool, or MBM booster tool. Summit, jegs, etc. sell it. You might not need it. You stick a small awl or jeweler's screwdriver in the hole and move the valve to center it. If the system is all closed now, I don't know if it will move or not, but I think it will if the MC cover is off. Years ago, like 2 people could do it with one watching the light, the other bleeding (and yelling). We just discussed this over across the hall on the Buick site recently. I'll look for links...
Dean, I don't think shimming the cylinder is a good fix. Here's more info: there is a cup-like seal/gasket that goes between the cylinder and the booster. In your #1 post you can see it. It's almost always missing, they were unavailable except NOS for a long time, but now available repro. That could fix the shim issue or not. There is a tool to hold the prop valve centered, it's like $8.00 or so. Look on ebay for booster valve tool, or MBM booster tool. Summit, jegs, etc. sell it. You might not need it. You stick a small awl or jeweler's screwdriver in the hole and move the valve to center it. If the system is all closed now, I don't know if it will move or not, but I think it will if the MC cover is off. Years ago, like 2 people could do it with one watching the light, the other bleeding (and yelling). We just discussed this over across the hall on the Buick site recently. I'll look for links...
Beat me to it. The M/C must be sealed against the booster with that rubber insert in place. That's what seals the vacuum on the forward side of the booster diaphragm.
i shimmed the MC out to see if thats why my front brakes drag, it is not, i saw no change w the MC shimmed. Ill inspect for that seal and its condition.
my valve is definetly off center, as the brake light is on. I cant tell by looking whichway its off. I have jammed on the pedal hard while driving several times and the valve has not recentered. I could get in there w an awl but that small hole makes any manipulation difficult.
most everything in the brake system has been replaced and seen little use but may be 10 years old or newer.
i replaced the rear axle and everything brake related in the rear about 5 years ago, i dont recall having any trouble bleeding them then, both front calipers were replaced more recently again no trouble w bleeding them.
ill have to examine all four brakes and rebleed trying to get that valve centered.
August 22nd, 2021, 03:25 PM
note the precision measuring tool 
