Inline Tube - In Proportion
Tips for choosing the correct valve for your A-body disc brake conversion or
restoration
Which valve do I need? There are so many to choose from and they
are all different sizes and the fittings change as well. This is why it is
difficult to pick out the right valve for you car. Some valves are used in
conjunction with others and many are singular units which require no other
valves. Most people call these items proportioning valves but very few are just
that. Some are distribution blocks and others are hold off valves. Below we will
go through all the different years and the applications.
Proportioning valves come in all shapes and sizes. These valves vary depending on the car you are working on. The problem is which valve should be used on which car. We asked the experts at Inline Tube and received the full scoop on the different years and the functions of the valves.
There are plenty of disc brake sets on e-bay, at the swap meet, on junk yard cars, but most people don't know what will fit their car. It is important to know the exact year make and model of the car the brakes came off of. Most of the parts cross over to work on a number of cars but knowing what to look for is the hard part. When ever I see a disc brake set up at the swap meet and I ask what year it came off and the standard answer is 72 Chevelle. When I look closer I can determine the exact year from the valves that are with the set. Although that 72 Chevelle will fit the 68 Chevelle this does not mean the car now has factory disc brakes. Lets start at the beginning.
The first year for disc brakes on GM A-body products is 1967. All cars before this point in time had four wheel drum brakes. Disc brakes were introduced as an option from 1967-1972. Starting in the 1973 model year front disc brakes were standard and front drums were no longer available. When disc brakes were developed so was the dual master cylinder, so all 1967 and newer cars had dual masters. Disc brakes required different pressures so the master was divided into two halves to provide pressure to the front and rear separately. This also meant that if a front line blew out the car would still have pressure to the rear brakes and vise versa.
The first thing we have to clear up, is that all 67-68 Factory disc brake cars came with dual piston calipers, if the car is a 69-72 it had single piston calipers. Most after market sets come with the later 69-72 single piston calipers but this is not technically factory correct for the 67-68 cars. The single piston setup will bolt on all the 64-72 GM A-body's. The first issue to converting a car to disc brakes is what kind of car is it. If the car is all factory correct and that is a concern, single piston calipers on a 67 or 68 car will certainly catch the judges eye and throw up a red flag. If the car is a driver or modified car no one will care if it has the single piston or dual piston calipers. The first question we ask at Inline Tube is are you looking for original appearance or are you looking for something that just bolts on and appearance may not be an issue.
The next issue is the proportioning valve since disc brakes were new in 1967 the valve was still being worked out for the next few years. The 1967-70 valves could comprise of as many as three pieces and with out all the pieces the valves would not work properly. In 1971 the valve was finally worked out and this design was used on most cars from 1971 into the 1980's. This is why it is important to know what are the correct pieces for your car. People generally refer to all blocks as proportioning valves. There are metering blocks, hold off valves, adjustable proportioning valves, residual valves, and what is the right combination? Once you get past the spindle and calipers the brake lines will drive the average guy insane. With the wrong caliper the hose may not fit. With the wrong valve combination the lines may not fit properly. The fittings sizes change with the year of the valve, and valves that look the same are not. Factory lines do not work with aftermarket valves and in some cases headers will hit blocks located on the frame. I have put together a set of photos to make anyone the expert. Next time you go to the swap meet you will know what to look for. Inline Tube also makes all the valves and the lines to work with them but you still have to know what kind of brakes are on your car. Remember most of these cars are over 30 years old and parts have been changed, just because the car has disc brakes does not mean they are factory.
Lets start in 1964, engine performance was rising and braking
needs had to start to change. On all 64-65 and 66 cars the only brake option was
either power or manual drum. No disc brakes were available. These cars had a
brass distribution block mounted at the frame with a single master cylinder line
feeding the entire system providing equal pressure to all four drums. The 64-65
block has a smaller fitting on the front to rear brake line (purple) and since
1966 cars all went to 1/4" front to rear brake line the 66 block has a
larger fitting (red). It is not uncommon for blocks to look the same but have a
change in fitting size from year to year. Most components are year specific.
Inline Tube has made the factory single master cylinder block and a new block
for cars that are converting to a dual master cylinder.
The dual block and lines shown here are ready to bolt on and
convert you 64-66 to a dual drum master cylinder splitting the front brakes from the rear so if the system ever goes down you only loose half the
brakes instead of all the brakes. If you are choosing to convert to front disc
brakes the adjustable valve will have to be cut into the front to rear brake
line to reduce pressure to the rear brakes. Four wheel drum uses equal pressure
while disc brakes uses more pressure to the front and less to the back so the
adjustable valve will correct the problem.
In 1967 all cars had the dual master cylinder and
a brass distribution block (pictured to the right). When you ordered disc brakes
your system had the additional disc brake hold off valve (pictured to the left). The function of this valve
was to hold off the front brakes for a split second and once enough pressure was
built up to activate the valve spring the valve applied
pressure to the front brakes. The rear brakes were to come on a split second
before the front. This brake method soon caused braking problems that took a
couple of years
to solve. The largest problem was the car had uneven braking causing the car to
want to slide out, just like doing donuts with the parking brake cables applied
on front wheel drive cars. This is great fun in a parking lot with no other cars
or obstacles around but not on the highway.
In 1968 the car body style changed and and not
much attention was paid to the braking system. The 67 and 68 systems are
identical although the mounting of the valves is a bit different and line
routings change with the different frame. The components and the functions of the
components are the same. 1968 also carried over the dual piston caliper for the
last year. The new disc brake option is now beginning to be noticed and orders
are increasing and 1969 is going to bring big changes.
In 1969 the problem free single piston caliper is
introduced and the refining process of valves is in the beginning stages. A new
larger hold off valve is introduced and is working much better with the caliper.
Now disc brakes and performance go hand in hand and while disc brakes is still
an option so is horse power and people are beginning to realize to go fast you
have to stop fast and they are now willing to pay for the brake option. The
fittings change on these valves almost every year so
they will not work with the brake lines from
different year cars.
In 1970 the single piston caliper is proved to be a winner
and is used up until the late 80's. The valves in the system is a different
story. Big engines cause cars to be front end heavy and current hold off valves
cause the front brake to dig in and the rear of the car lifts causing the rear
brake to skid. So the 70 Hold off valve is changed again but this time they
weaken the valve spring and add a new rear hold off valve. So now there is a
hold off valve in both the front and rear brakes and the car is stopping nice
and even just the way it should. Most don't even realize the rear valve is on
the car, it is located at the end of the front to rear brake line where the rear
hose goes through the frame and is an easy way to tell a factory disc brake car.
In 1971-72 disc brakes is gaining popularity and most
performance cars are equipped with them. The caliper is working great and now
the 70 valves are all combined into a one part valve that is call a
proportioning valve (pictured to the right). This new 71 valve works great and is used until 1988. This
is also the valve that is first choice of the disc brake conversion companies
and that is why it looks so familiar to us.
In 1973 all A-bodies now come with disc brakes standard and drums are no longer
offered. We all could of used disc brakes to be standard a few years earlier at
the peak of performance in 1970. Now the gas crises is among us and performance
decreases but disc brakes are hear to stay.
These hold off valves were used on 1967-70 disc
cars and changed just about every year. Starting on the left is the 67-68 valve
,the middle valve is the 69 valve, and the right valve is off the 70 cars. Notice
the fittings change on the valves so they will not work with the brake lines
from different year cars. The ones with the large nut can be rebuild but it often costs more than
buying it new.
In the 70's these valves were made in both the cast iron and brass depending on
the year and the vendor. The cast
part did not hold up very well to the elements, the brass valves clean up nice
and are easily reused. The pin style wire lead was used until 1977 and the
plastic lead housing is 78 and newer. This valve was used on most GM cars up until 1988
and is a great valve for disc brake conversions. It is available from inline
tube in brass with both wire lead options although most cars use the metal pin
style.
The adjustable proportioning valve is usually
used to tune in brakes. The valve goes in the back brake line affecting only the
rear brakes. The valve reduces pressure to prevent the tires from locking up. If
the car is running wider tires in the back and narrow tires in the front there
is now more surface area in the back causing the rear drums to do all the
stopping. This valve reduces pressure to even out the brakes. This valve is also
ideal to be used with the distribution block shown earlier on four wheel disc
cars. The distribution
block provides equal pressure to all four wheels and again this valve goes in
the rear line to reduce the pressure to the rear brakes. Ideally the front
brakes should be getting 70% and the rears 30%. On four wheel disc applications
50/50 is normal.
Inline Tube offers valves, brackets and master
cylinders for any kind of conversion you choose to do. If your car is going for
that factory concourse correct look (pictured right) or is modified (picture
left) they can help. Inline Tube offers
kits for cars that are converting to dual master drum brakes, Disc / Drum
conversions or four wheel disc applications. In the modified version the valve is neatly tucked under the master and away from header heat on the GM
A-bodies. The valve is available in the Disc/Drum version and the four wheel disc version. The brass construction prevents rust and the bracket holds it
securely in place. Inline Tube also offers disc brake conversion brake line sets
for all valve applications.
Inline Tube can help with all your disc brake and valve needs. Valves are
available new for all applications and your unanswered questions are only a
phone call away.
