FREQUENTLY ASKED QUESTIONS ABOUT POWER BRAKE BOOSTERS:
Q1) What do boosters actually do?
A) A brake booster gets all the air sucked out of it by the running engine. When you apply the
brake pedal, a valve in the booster allows air at atmospheric pressure back into ONE side of
the big chamber. The unequal air pressure moves a diaphragm and pushrod inside the
chamber. This adds to your foot pressure on the brake pedal, and you stop with less effort.
Q2) What types of Boosters are there?
A) There are essentially only two types: HYDROVACS and MASTERVACS.
Q3) What is a Hydrovac booster?
A) We need some history here. Hydrovacs were the early type of brake booster, fitted around
1954 on some cars of that period. They were plumbed into the brake line immediately after the
master cylinder, effectively boosting the pressure to all four wheels. Sadly, these early
hydrovacs are very complicated in that they combine several vacuum and hydraulic valves and
cylinders which, over the years, have aged, are worn and corroded. Proper restoration involves
machining the body and fitting special stainless or bronze sleeves in the damaged bores.
Check the left and middle photos above.
Q4) Then what are Mastervacs?
A) Now check the right-side photo above. You will see a Mastervac is a much simpler unit. This is
because it is a stand-alone unit, it bolts to the master cylinder, and is NOT a part of the
Q5) What is the significance of this change of design to us?
A) Bottom line, HYDROVACS cost more to restore to good working order. They are older and
more delicate. Their hydraulic cylinders are almost always corroded and require precision
Q6) Are the Brake Doctors familiar with all boosters?
A) Yes we are. This includes Mastervacs, Hydrovacs, Treadlevacs and remote boosters. We've
been in the brake business since 1956 and we have seen them all.
Here's a list of the makes you may find:
Aisin; ATE; Bendix; Clayton Dewandre; Delco; Girling; Girlock; JKC; Knorr; Lockheed;
Midlands and PBR just to name a few.
Q7) Are all boosters repairable?
A) We can restore most boosters, even some sealed units that come crimped together and which
were never intended to be serviced.
We keep a comprehensive range of brake booster parts that are sourced worldwide to ensure
the overhauling of as many boosters as possible. This includes boosters for American vehicles
such as Cadillac's, GM's, Mustangs and Galaxies; English vehicles including Jaguar,
Aston Martin and Bristol; and European vehicles like BMW, Alfa Romeo and Mercedes.
Q8) Are all boosters tested after restoration?
A) Yes they are all tested on a dedicated test rig for output, vacuum and hydraulic performance.
Q9) What is the likely cost of restoration?
A) If your booster is a Mastervac, we can quote with confidence. If it’s an older Hydrovac, we can
give you a worst-case scenario, with a promise to do only that which is necessary for a
guaranteed job. Just putting a repair kit in one of these units with worn, corroded or oversize
bores is futile. So the cost is ultimately determined by the number of cylinders bores that need
to be sleeved, which is only apparent upon inspection.
Q10) Can you suppy repair kits so I can overhaul my own booster?
A) In most cases we can, but for everybody's safety we prefer not to.
Q11) Why is that?
A) The four main reasons are:
a) Some boosters or power brakes were designed as a sealed unit, never to come apart.
Special tools and equipment are needed to dismantle them.
b) New seals will fail prematurely if fitted to corroded or scored slave and control cylinders in
need of sleeving.
c) Kits only cover major components. You may not recognise other critical worn, damaged or
d) Your best efforts may be in vain if you can't test the unit for vacuum and hydraulic leaks, and
overall performance, before refitting to your vehicle.
For these reasons, we suggest you send your unit to us for a professional tested outcome.
Q12) I'm not sure if my booster is working properly. How can I test it?
A) Here's a simple test. With the motor not running apply the brakes 5 times and wait one
minute. Then apply the brakes as normal holding your foot on the pedal. Now start the engine.
If the booster is working properly, the brake pedal will sink a little as the booster applies extra
pressure. Now turn the engine off. Your brake pedal should stay down. If your pedal slowly
comes back, you have a vacuum leak.
Q13) What are the most common faults with brake boosters?
A) Fault 1...Ruptured Diaphragms in both Hydrovacs and Mastervacs. Result...Engine will idle
faster and may stop when applying the brakes.
Fault 2...Seal failure in Hydrovacs only, causing an internal fluid leak. Result...Fluid will be
sucked out of the booster and into the motor and will burn and cause eventual brake failure
once the master cylinder is empty of brake fluid.
Fault 3...Reduction, or loss, of vacuum supply in both Hydrovacs and Mastervacs.
Result...The reduction or loss of vacuum supply will result in a hard pedal and poor braking.
To test, remove the vacuum hose from the booster and start the engine. A vacuum gauge should
read 20Hg on the overrun. Just putting a finger over the hose end is not conclusive.
Depending on age and condition, the hose may be heat and fuel affected and could collapse
when the air gets sucked out of it. If you have any doubts, replace the vacuum hose and examine
the external check valve. You should be able to suck air in through the valve, but not out.
Power Brakes facilities and collective experience are available to identify and correct all these
Q14) Do you offer a guarantee on Power Brakes products and brake boosters?
A) Yes we do...just click on the "Your Guarantee" button for details.
Pictured Is A VH44 Hydrovac On The Test Bench.