Rear Brake Valve Notes   

Nov-2020  R. Kwas [Comments Added]

Note:  Some of the info here is is German or Swedish because it is the language of the original source material, with translations of the necessary parts.


Reference Information
    Operating Pressures
    Workshop Bulletin

    Volvo Brochure


Rebuilding Operation

Brake Valve Teststand


Background:  This part was installed by the factory into Amazons and 1800s produced after August 1965.  It is intended to reduce the risk of a rear brake lock-up due to weight-shift, during maximum or panic braking conditions.  When Brake system hydraulic pressure exceeds 430PSI (adjustment and vehicle model dependent), it reduces the pressure to rear (only) brake wheel cylinders thereby decreasing their effectiveness and reducing the chance of a rear lock-up.  

This is what one looks like, without undercoating plus 50 years of under-car-accumulation!


A Rear Brake Valve as received from favorite used parts supplier Chris Horn.  Some surface rust is present, but most of the surface is well protected by simply rust colored undercoating, as the clean area shows, where the undercoating has been chipped/scraped away.  Penetrating oil is applied to fittings in preparation for disassembly. 


Plumbing fittings are removed.  Super-groovy tubing wrench is used to loosen fittings as with the maximum loosening torque necessary, its important to be applying this to all six flats of the fitting (although with the brakeline simply cut off, a normal closed-end (aka:  box) wrench would certainly have done it too...!).  Again, 3/8 24TPI fittings with bubble-flares and short tubing nuts are found upon disassembly (as used in the rest of the 122 Brk Sys).  Ref:  Brake Plumbing and Fittings    


When scraping the undercoating off in an effort to locate where the adjustment force has been "punched into the housing" (info from Volvoniac Forum Member:  Rudi) ["Druck ist eingestellt mit der Einstellschraube, die man beim Überholen daher so lassen soll wie sie ist, und eine entsprechende Zahl ist im Deckel eingeschlagen." My translation:  Operating Pressure is set by the Adjustment Screw, whose setting should be left unchanged when rebuilding, and this pressure is punched into the End-plug. ], cleaning away the undercoating reveals the numbers 467G  -20, punched into the Plug(1).

Brake Valve Plug(1) with pressure setting punched in.  ...and at 1 1/4", I already have the proper socket to remove that Plug!

Item Reference Nos are from Volvo Workshop Bulletin below. 

At the other end, it is clearly apparent, locking Nut(12) on Adjusting Screw(11) has been epoxied into place after adjustment.




Reference Information: 

Operating Pressures: 

P1800 Brake Pressure Regulator Rear Axle / P1800 Bremskraftregler HA

Link to Volvoniacs forum Thread:

From that thread: 

For reference, atü is number of 14.7PSI atmospheres, so 20atü = 20 X 14.7 = 394PSI 

Note:  Opening Setpoint pressure, is a function of the Rear Brake Wheel Cylinder Diameter 13/16" or 1" was fitted, and that is in-turn a function of if the Vacuum Assist unit was installed. 


Workshop Bulletin:  


Adjusting Screw(11) and Locknut(12) should not be touched by mere mortals without the fixturing with which to check and verify the adjustment!  Refer to:  Jürgen's Test stand below!  


Volvo Brochure: 

Panic-Free Brakes - Volvo's explanation of the purpose and function of the Rear Brake Valve 

[Thanks to Volvo-colleague Fredrik Lofter for providing this translation of the Volvo Technical Information:] 


Top picture text: Each brake circuit in Volvo’s cars is fitted with one of these reduction valves that prevent the rear wheels from locking before the front wheels when braking hard.

Panic-free brakes:  An average car has its weight distributed about the same on the front wheels and on the rear wheels. Volvo 142/144 for instance has 52% of the weight on the front wheels, 48% on the rear wheels. When braking however, the load increase on the front wheels while it at the same time decrease on the rear wheels. The load on the front wheels is bigger the heavier the braking is. The left picture shows how the load is changed with an increased braking. (Braking intensity is measured in g – 0.8g corresponds to a very hard braking).

Left picture text: The harder the braking, the bigger the share of the car’s weight on the front wheels. The arrows are proportional in relation to the load at different hard brakings (0.2g = moderate, 0.5g = pretty hard, 0.8g = very hard braking). The circles illustrate the corresponding advancement (as in forward moving) of the center of gravity. The load on one wheel is of great importance for that wheel’s braking ability. Big brake force on a wheel with small load tend to lead to that wheel locks. Therefore it is normally the rear wheels that lock first when braking hard. Because a locked wheel has poor braking ability it is ideal to construct the braking system in such a way that locking of the wheels is avoided. In other words the brake force should be adapted for each wheel’s load. Volvo has solved this problem with a reduction valve in each of the brake circuits (the picture to the right). When braking moderately the brake fluid flows freely through the valves. When braking hard the valves are closed and the rear wheels get a smaller/reduced share of the brake force. The construction is made so that the change in the brake force share very closely correlates to the change in the weight distribution at different braking intensities. The result: Best possible braking effect even with abnormally hard braking. VOLVO

Right picture text: A reduction valve (R) in the brake circuit/line prevents involuntary locking of the rear wheels before the front wheels when braking hard.


A detail of the above...275G is clearly discernable (275 /14.7 = 18.7atü, so this is in the typical operating range for some chassis) ...because notice also that the Volvo information is from the 140 era (from the side-view shown in the graphic), where the set-values would have been different from the earlier (lighter, and rear Drum) 122 and 1800 chassis.



Rebuilding Operation (Swedish):


Rebuilding Operation:

Until I perform this operation myself, and document it in detail, here are a few pix and tips by Jeff Guinn.

Pictures reposted with his kind permission. 

For a brilliantly simple, controlled technique of safely releasing the significant stored preload in the Spring(13), two of the four bolts holding the Valve Assembly to the Bracket and Spring Housing(10) are first removed, and replaced (diagonally) with two longer (2") bolts.  The remaining shorter bolts are then removed, after which the longer bolts are loosened slowly, thereby releasing the stored energy of the Spring gently with no danger to your fingers, others in the shop, or the cat watching... 


Parts are laid out for inspection after a drama-less disassembly thanks to Jeff's effective spring-force controlling technique! 


The bullet shaped device highlighted is an "assembly cone" aid, included in the Rebuild Kit, which allows a progressive, gentle, non-violent (or damaging) placement of the smaller O-Ring.  After this special tool is used for its intended purpose, it can be forever stored in your toolbox, where the next guy who owns the toolbox will forever ponder what it may have been used for...that is why it might be good to mark and ID it! 

A good assembly technique makes use of the "assembly cone" and successively larger sockets to push the O-Ring into position. 

The wide end of the O-Ring faces away from the spring assembly. 

"Assembly cone" in use...


Torque value for Plug(1):

10-12 mkp (Meter KiloPound)  X 7.2 = 72 - 87Ft/Lbs

Reference Converter:


Brake Valve Teststand:  Only if you can set up one of these to check input and output pressures accurately, may you touch and change the Adjusting Screw(11) and Locknut(12)!  Function is straight-forward, but because of the serious forces involved, teststand must be "man-sized", and secured to a serious bench...notice the weldment construction, and lever with HUUUGE advantage, with which to simulate a good, strong legpush into the Test Master Cylinder, resulting in the necessary 400PSI in the line, to be able to observe opening point.  Tip:  Leverage is your friend!

Jürgen's Meggle's picture used with his kind permission.

Jürgen's beefy benchtop teststand consists of a Master Cylinder activated with a handlever (note complex fulcrum for increased advantage!), RBV under test, plumbed to incoming and outgoing Manometers.  Catch-container at the right is for catching fluid when bleeding the system, and there is a bleeding and plug provision in the line just before it, where the line transitions from black to yellow. 


Source and PN of rebuild kit:

VP Autoparts, part number 273090, [$31, when I last checked].


External material is attributed.  This article is Copyright © 2020-2023.  Ronald Kwas.   The term Volvo is used for reference only.  I have no affiliation with this company other than to try to keep its products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here.  The information presented comes from my own experience and carefully considered opinion, 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, and 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 odd metaphor and probably wise-a** comment. 


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