1800E Brake Lock-up due to Lack of Free-Play...between Brake Booster and Master Cylinder

Jul 2022 R. Kwas [Comments added]

I would expect the info herein to be applicable to all models with tandem Brake-Boosters and Master Cylinders, when replacing MC, or disassembling any part of the Pedal/Booster/MC assembly.


After renewal of Caliper and Flex-lines on his 1800E, as well as bringing Brake Booster back to functional and finally contributing (by correcting a Check-Valve found to have been previously installed in the wrong direction!), this owner found the Brakes now locking all wheels during a test drive, and posted to looking for help: 

Excerpt of FB posting by Mike S.:  ...72 1800E back on the road after a lengthy garage stay, so all the calipers were stuck so I rebuilt them all with replacement pistons, replaced all 6 brake hoses, that all seems ok, on my test drives brakes worked, not great, after a short trip the brakes were dragging to the point of stopping the car when I pushed the clutch in, the brake pedal was like a rock at the top of its travel. I have not messed with the master cylinder or pedal assembly and they used to work fine years ago. One thing I found that might have bearing on the problem is that I found the check valve for the booster was (since I’ve had the car) in backwards so I turned it around. " 

My response:  "If Calipers and Flexlines have been replaced [and Brake issues persist], in addition to suspecting Master Cylinder [This was subsequently replaced, and did not correct, but may have actually contributed to the symptoms.], I also suspect adjustment and interaction of Master Cylinder Actuating Rod into Brake Booster (has this been touched/changed?)...if this is out of position/adjustment, it can result in effectively continuously applying the brakes a little, and Booster (now that it's working!) does the rest...and that sounds like your symptoms..."

Mike's findings:  "...a solution, not the fix, thinking it was the master cylinder not releasing the pressure, and not knowing how to adjust, I loosened the nuts on the master and shimmed with washers… problem found[!!], brakes no longer stayed pressurized. Now to find out how the adjust the activation rod between the master and pedal/booster..." 

My Response:  "Spacing the MC away correcting the issue, confirms interaction of MC and Booster, and free-play needs to be adjusted...I suggest you decrease length of Thrust Rod (6) and threaded part of Yoke (7) by the thickness of the spacing washers you added. " [It would seem that Volvo did this by way of a Transfer Pin and Spacer Washer(s)/Shim(s).  Read on!]

Source of graphic: Service manual 1800E 1971, under Group 54 Auxiliary Brake System.

Conclusion:  Thrust Rod (6) must have some clearance/free-play to MC Piston when Brake Pedal is at rest. 

What Mike found:

Mike Sparkes pictures used with his kind permission.


Actuating Rod fine length adjustment by way of Spacer Washer(s)/Schim(s) and a Transfer Pin.


Close-up of Transfer Pin


Here is the marked-up factory drawing from the 1800E Service Manual to add and capture the pertinent info. 

It would seem that the design and manufacturing dimensions leave a generous amount of free-play which must be minimized, and this is done by way of the Spacer Washers/Shims on the Transfer Pin  This also means, that when replacing the Master Cylinder, the interfacing dimensions should be carefully checked, and if necessary the SW/S must be adjusted to leave a slight gap (no SW/S at all would result in too much of a gap/free-play, which would translate into a longer Pedal), and a stack of SW/S too think would result in a negative clearance (MC would always be depressed a slight amount), leading to the "locked [or partially applied] brakes"  symptom described above. 

Of Course, the same applies when replacing the Pedal assembly or Booster...interfacing dimensions should always be double-checked to assure one is replacing likes-with-likes...and they are equally adjusted! 

I would say this is another example of "Two Wrongs make a Right"! ...in that, when the Check-Valve was installed in the wrong direction, the Brake Actuation rod was not "helped" into the MC, so the fact is that it did not have the required free-play did not result in any perceptible change...only after the helping action of Brake Booster was restored, did the lack of free-play only make itself known through symptoms.


Reference Info: 



External material sources are attributed.  Otherwise, this article is Copyright © 2022.  Ronald Kwas.  The term Volvo, is used for reference only.  I have no affiliation with this company other than to present my experience, and highly opinionated results of the use, care and servicing of their products, for the purpose of helping other owners keep their vehicles on the road, safe and reliable.  The information presented is my own, and can be used or not, or ridiculed and laughed at, or worshipped, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles, and future! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you don’t, you’re just a lazy, scum sucking plagiarist…so The Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the unique metaphor and probably (likely) wise-a** comment. 


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