M/C and
Hydraulic Valves
Hydraulic Valves – to develop brake biased or minimize rear wheel
lock up
Master
Cylinder
·
Primary is mechanically (manually) operated
·
Secondary is hydraulically operated
·
Each has 2 cups, a primary and a secondary
·
Primary pushes fluid towards wheels, it is at the
front
·
Secondary – all it does is hold reservoir fluid
Compensating Port – open
path from caliper to reservoir
·
When piston is released, must leave port open
·
Disc brakes won’t release if port isn’t open
·
For heat expansion and to release disc brakes
1.
Take reservoir off
2.
A stream or squirt should come out of top of
reservoir when brakes are applied
·
If brake pedal sinks, (+no leaks, + no hydroboost) –
primary seal
·
Brake fluid leaking on pedal – secondary cap in
primary piston
Diaphragm – in
reservoir, seal acts as a bellows
1.
Keep sealed from moisture
2.
To prevent vacuum, act as a bellows
Quick Take Up Master
Cylinder
·
New design on calipers, to increase fuel economy
·
Disc brakes had a bit of drag, not much run out
(1978)
·
Square cut seal is set in angle – low drag caliper
·
Problem – square cut seal pushes caliper back too
far; this causes low brake pedal
·
During initial brake application (low pressure),
fluid goes through a larger orifice, higher volume of fluid going to caliper,
takes up slack faster
·
Then valve closes to act as a regular master cylinder
when pressure builds up
Combination Valve - housing of 2 or more valves, usually 3
·
Mounted near master cylinder usually or below feet
under frame
Metering Valve – hold off
valve “FRONT” Brake system
Pressure
0 – 1200
·
Operates front wheels
·
As soon as we hit brakes, valve closes
·
Holds off pressure to the front until back has a
chance to apply
·
Anytime you let off brakes you must have lines open
to the reservoir to allow for heat expansion
·
Sometimes when bleeding the valves, if you can’t get
it to let out of the front, metering valve could be problem. You can bypass the valve manually to open.
Proportioning valve – goes to
rear brakes
·
Operates at relatively high pressure, starts @ about
500 psi
·
Doesn’t do anything until a panic stop, starts to cut
off rear brake pressure
Metering
Open 15 psi and below – approximately
to allow for heat expansion
Closed 15 psi and up
Open 75 psi - pressure
it takes to overcome spring tension in rear drum
|
Proportioning |
|
|
M/C |
Rear
drums |
|
500 psi |
500 psi |
|
550 psi |
540 psi |
|
650 psi |
600 psi |
|
750 psi |
650 psi |
Height sensing
proportioning valve
·
Positive frame angle, up in the back
·
More of a load, needs more braking
·
More of a load reduces ride height
Pressure differential valve – right
below switch for brake warning lamp
·
Always in middle of 2 halves of system
·
If a line on one halve brakes, this valve will move
over and ground switch
·
Operates at 75 psi difference in pressure between 2
halves
1.
Self centering – has springs. Light will only go on when brakes are applied
2.
Non self centering – has to be hydraulically moved
over. After fixed, it will stay where it
is put, so light is on.
When rear locks up, it tries to fish tail around.
- rear gets in
front of you – front end is braking – rear is moving faster
1987 – 1993 – Trucks –
1/2 ton pickup
Chevy RAWL
- Rear Wheel Antilock
Ford RABS
·
Antilock light is same as all other brake (red)
warning
·
Computer in
ABS has a resistor, doesn’t light as bright
- Apply park brake and it gets brighter, ABS problem
Residual Pressure Valve (Aka, Check Valve)
Residual pressure
– pressure that is going to reside in the brake system
·
DRUM BRAKES ONLY!
·
10-20 psi
·
Keep seals from leaking in wheel cylinder
·
Mounted in master cylinder
·
Keeps seals up against side wall of cylinder
·
Because side of seal are at angle, the more pressure
inside cylinder, the better it seals
What Keeps the Seals in
Place?
- residual
pressure valve
- cup
expanders/springs
Brake lines – steel - not interchangeable
·
ISO
(International Standards Organization)
·
Double flair – metric, English
·
Anytime you have any line that is broken, replace it
all- of that line!
·
Start at back and work to front
·
Reuse nut at proportioning valve
·
Tubing Benders
·
Flare Nut Wrench/Line Wrench
·
When removing the line from wheel cylinder, the line
can twist so use a special tool
Opening the Bleeder
·
6 pt. Box end wrench
·
A deep socket may break bleeder off
·
If you break bleeder off, replace cylinder
·
To get loose, use heat and tap w/hammer. Do not use heat if aluminum
·
Not petroleum, will get into system, expand rubber
·
No penetrating oil
Lock-Tite
·
Many times requires heat to break loose
·
Don’t put lock-tite on any “sending” devices – gauges
Example: Oil pressure – can cause higher resistance
H.P.accumulator – gives us
some assist
·
If engine quits, still work, failsafe
Electric/hydraulic – electric
motor
·
Powermaster GM – Diesel engines – no vacuum
·
Can also be in non-diesel
Brake Boosters
·
Vacuum assist
·
Hydraulic – hydro boost, runs off power steering
·
Electric/hydraulic – Ex.: power master GM
Vacuum
·
Hose from intake manifold – one way check valve
·
Highest vacuum at idle – in of vacuum
·
Constant in measuring vacuum is mercury
Halve the vacuum – halve the assist.
Vacuum Test - to run on booster
1. Check for leaks – 3 minute leak test
a.
Shut car off
b.
After 3 minutes, pull vacuum hose from booster (or crack open)
c.
Hissing means air is coming in
d.
This noise means that the booster can hold vacuum for this amount of
time, so the leak cannot be significant
2. To tell if it
works – Booster test (for all types)
a. Turn car off
b. Pump brake pedal until hard, evacuate all
assist
c. Hold foot on brake pedal, start car
d. Should drop slightly to floor (about 1 inch
or 2)
Don’t recommend using a hand vacuum pump.
Engine is best vacuum pump!
Brake Bleeding
·
getting air out of system
·
when you have air, you have a spongy pedal
·
if you have any leaks coming out, you probably have
air coming in
Brake
*****Never
back-flush into the master cylinder.*****
· Some people
will rebuild wheel cylinders when doing a brake job
1.
Suck out fluid from bleeders, until fluid is clear
for bleeding
2.
Wipe out bottom of reservoir
3.
Replace with fresh brake-fluid
Bleeding
1.
Breaking open a bleeder valve- careful not to break
·
Put rubber hose on bleeder into a glass jar
2. Get fluid to
move out:
a. Gravity – open bleeder, drip out; make sure
bleeder hole
is not
clogged
b. Vacuum
c. Pressure – put pressure on reservoir; not
used often
d. Pedal – pump up brake pedal, hold; you do not
have to have car on
if you
have a booster
3. Before you let
vehicle down, clean up any brake fluid.
Tires can fling.
Sinking brake pedal
(slowly)
·
If no hydro-boost and no leaks to floor
·
Master cylinder
Pedal Bleed
·
Only go half way, don’t extend all the way
·
Crack the bleeder valve
·
Push half way on brake pedal
·
When you go all the way it can
damage master cylinder
ABS
·
Anytime you replace hydraulic system after you bleed,
you must also bleed module
To do this simulate ABS situation