New doubt about brakes.

[Fernando Mendes]

To all folks.Who has experience in this kind of doubt.When I press my brake pedal,notice that the rear wheel tandem became heavy,not free.It only will be free after some minutes.I think it is problem with wheel cylinder that aren't returning or not releasing.I spoke today with a friend that says to me it can be problem with brake hoses clogged or wheel cylinder.But my hoses are new,I don't think in this way.What can it be.

[Fernando Mendes]

When I press AND RELEASE my brake pedal.........of course.

[Chris_M]

Fernando,
I had exactly the same problem. First I thought the brake cylinders were clogged or the brake shoe return spring was weak. So I checked everything. Springs were strong and in good order. Cylinders were a bit stained but not much. I cleaned them thoroughly. The problem still existed.
So I checked the brake hoses. My truck is a last series made vehicle. So the brake lines and hoses are completely different to its predecessors. The two rear axles have only ONE break hose each. They are not fixed to the wheel backing plates. These are all steel lines. The two hoses are installed at the center of the axles near the differential housings. The hoses are running along the suspension near the housings and are CLAMPED on it. And that was the problem! The preowner screwed these clamps much too tight and squeezed the hoses. When applying the brake, force was strong enough to squeeze brake fluid through the hoses. But upon release of the brake pedal the fluid could not flow back. Rear brakes still applied and released very slow as the brake fluid "dripped" back. Front brakes were completely released of course. I screwed open these clamps a bit and I could hear the brake shoes returning to open position with a gentle screeching noise.

Regards,

Chris

[Captain Crank]

Good post Chris.

I had the same issue with the front brakes on a half track.

When stepped on, one brake was hanging up and then releasing very slowly.

Turns out when a trucker had chained it down he squished the brake line next to the spring perch with the load chain.

Enough pressure to engage brake but had to slowly bleed back.

Check all your hard lines Fernando.

[Fernando Mendes]

Thanks guys for your help.I think is very important we are speaking about our experiences in the past.I have one brake hose per backing plate.A friend that I spoke yesterday here in Porto Alegre(I am travelling now) sad to me that is very sample to check this problem.1)Jack up each rear wheel,one at a time.2)Other person press and release the pedal.3)If this wheel turns by hand it is all ok(with cylinder,return spring,hose,etc).4)If this wheel don't turns by hand.a)open the bleeder screw.(1)If the brake liquid don't goes out is a wheel cylinder problem.(2)If the brake liquid goes out isn't a wheel cylinder problem.NOTICE:We can't say in these two last cases that the problem is with the hose,because the liquid passed through it.What do you think?

[Fernando Mendes]

Hi Doctor Deuce.Yesterday when I jacked up my rear wheels,one at a time,to be sure,if the brake wheel cylinders were free or not,after applying and releasing the pedal,with the engine stopped,I was sure that all wheels were free.I could turn them with my hands about only 30 degrees,although the transmission shifting lever was in neutral position and auxiliar range too I can't turned totally free(360 degrees).What can I do to get them totally free to turn?After when I started the engine and applyed the brake pedal and release,tryed to run in first shifting gear and felt again the heavy running,not wheels free.Can it be hydrovac problem.What kind of sequence test to do to know if it is the solution?

[dr deuce]

You need to jack up one wheel on both axles at the same time. The two rear axles are geared together through the transfer case. That is why you need to match the rear tires.

[Fernando Mendes]

And about the hydrovac?The brakes are holding only when the engine is running.Can we say that the problem is in the hydrovac? not in the wheel cylinders?

[dr deuce]

It could be the master cylinder telling the hydrovac to stay on.

Try opening the line from the master to the hydrovac and see if it releases.

[Fernando Mendes]

Doctor thanks for your help.
1)Opening the bleeder screw in hydrovac......a)If it releases.....possible can be what???? b)If it not releases.......possible can be......???

[dr deuce]

If you release the bleeder on the master and it releases, it is something with the master or possibly the booster.

If it releases with the hydrovac bleeder, it is the hydrovac IF you have done above 1st.

If neither, probably a hose acting as a one way check valve

[Fernando Mendes]

My master don't have bleeder.My CCKW serial number 309623-2 open cab don't have booster.I don't feel difference applying my foot in the brake pedal with the engine stopped and with the engine running.I suspect there is something wrong in my intake manifold valve or hydrovac.I will try to read enough the TM10-1563 to understand all its functioning.

[Fernando Mendes]

Look what I found in the page 12-19 from TM10-1563 today.A simple test to be sure if your hydrovac is or not functioning.
BRAKES.......11.HYDROVAC UNIT..............C.Testing power brake equipment.Before testing brake equipment,make sure that no leaks exist in the vacuum system:1.If engine is running,it should be stopped and brakes applied a few times so that supply of vacuum in lines and power cylinder is exhausted.2.Make a light brake pedal application with LEFT foot and hold the pedal in this position.3.Turn on ignition switch and start engine.4.As engine is started ,pressure necessary to hold the pedal will be partially relieved IF vacuum system is functioning.If there is no release of pedal pressure after engine has been started,this indicates that hydrovac power cylinder is NOT functioning.Refer to "vacuum lines","relay valve" and "power cylinder" for further tests.

[Fernando Mendes]

http://www.youtube.com/watch?gl=DE&hl=de&v=keChCevuKk8

[Fernando Mendes]

Page 12-19 from TM10-1563.........12.RELAY VALVE.This unit is attached to rear of power cylinder as illustrated in Fig.33.Refer to Fig.1 for hydrovac unit installation,also hydraulic and vacuum lines connections.Purpose of relay valve is to regulate and control vacuum and atmosphere pressure on both sides of power cylinder pistons.When brake pedal is depressed hydraulic fluid displaced by master cylinder builds up sufficient pressure to cause diaphragm linkage to open and close VACUUM and ATMOSPHERIC valves in accordance with requirements.Detailed description of the four functions of relay valve are given below:1.RELEASED POSITION-When brakes are not being used,relay valve is in released position,as shown in Fig.35................................As long as valve "A" remains closed,and valve "B" is open,engine manifold vacuum exists on both sides of power cylinder pistons. 2.APPLIED POSITION-When hydraulic pressure at relay valve is sufficient to cause piston to move diaphragm.................This permits atmosphere to pass through relay valve and to one side of power cylinder position.This pressure of air causes pistons to commence their power stroke and force push rod against hydraulic cylinder piston.3.HOLDING POSITION-..............Relay valve remains in this position with both atmospheric(A) and vacuum (B) valves closed and power cylinder pistons do not move in either direction,until, hydraulic pressure at relay valve is changed to open one of the valves.This action provides complete control of the power cylinder operation and prevents cylinder applying more brake action than is desired.4.RELEASING-When foot pedal is released,hydraulic pressure at relay valve drops,.............so that both sides of power cylinder pistons are again connected to intake manifold............Vacuum on both sides of power cylinder pistons is thus equalized and brakes are released instantly by their retracting springs.

[Fernando Mendes]

Page 12-21 from TM10-1563,RELAY VALVE...................A.Testing Relay Valve-To determine whether relay valve is functioning properly,the following procedure may be used:1.Disconnect atmosphere line from relay valve inlet and vacuum line from power cylinder center plate " Tee ".Light air pressure (not more tham 10 lbs.) should be applied through atmosphere inlet.If air pressure is retained for a reasonable length of time it indicates that valve is seating satisfactorily.If leak is apparent or AUDIBLE it may be around one of the GASKETS,instead of at atmospheric valve.2.Vacuum valve may also be tested at this time:except,that brakes must be applied to close vacuum valve while attempting to force air past vacuum valve seat.

[John V Cliche]

Glad to see you found your TM and taking the time to read it as well.



John

[Fernando Mendes]

yes John, I am learning enough with this readings.We can do a resume and see best the various cases that occurs inside the relay valve and power cylinder,same as with the tests.In next friday I will do the tests and see what is wrong in the system,like valve leaks(audible),change hydraulic fluid metal gaskets(see).Thanks.

[Fernando Mendes]

http://img138.imageshack.us/i/imagem330j.jpg/
My hydrovac unit isn't from chassis number 220521 through 321077.Who knows which kind of truck it is?See this pic.It has the slave cylinder and relay valve in the same side.My CCKW 353 is chassis number 309623.

[Fernando Mendes]

http://img80.imageshack.us/img80/7905/imagem335.jpg
http://img638.imageshack.us/img638/504/imagem336.jpg
I opened my hydrovac,it was with more pits of rust inside the cylinder.In the first step,for a while, I rectified inside(pic),after I filed to give a smooth and brightness(pic).