k5 brake problem 2

[cerryblazer]

thanks guys for answers!
so i told you when i push down brake pedal i hear as air come out under dash.
today i checked the system.
1° - i depressed brake pedal several times with engine stopped. with pedal depressed i started the engine and pedal gone down, so i think this test has been good.
2° - i started engine and lei it run for 3 minutes then i shuted it off, i depressed brake pedal several times and sinceraly i realized that pedal reserve distance has not increased every time brake pedal is depressed, or better not so much! (maybe 'cause brake pad are used)
3° - i realized that when i depress brake pedal the reservoir assembly rises from master cylinder body, but not simple a littel, it seems to escape from holes, precisly the part near the cab body!

 

[dremu]

It does sound like your vacuum booster is leaking -- the hissing noise is a bad sign.

Your point no. 3, when you're talking about the reservoir moving away from the master cylinder, I'm confused. The reservoir should be attached to the master cylinder ... but the master/reservoir might move away from the booster, if the bolts are loose or something is wrong.

Just to make sure, let's use a picture, in the bottom left corner:

http://www.lmctruck.com/ICATALOG/CC/full.aspx?Page=158

#2 is the vacuum booster, and if you have air leaking, that's what would leak.

#1 is the master cylinder (in that picture, the reservoir is cast as part of the master cylinder, but some have a plastic reservoir on top of the metal master cylinder.)

If anything is moving, I'd check first the two nuts that hold the master onto the booster (the two studs sticking out in the center of the booster in the front), and then check the various nuts and bolts holding the booster onto the firewall (there's six or eight on the back of the booster.)

If the master or reservoir were bad or leaking, you whould have brake fluid dripping out; if it's an air leak, it has to be the booster (or the check valve or the rubber hose.) You might try and locate the hissing noise; you can get "mechanic's stethoscopes" like this

which let you listen for where a sound is coming from. I've rarely seen boosters go bad, though it's possible; I'd suspect the hose and/or check valve first.

Hope that makes sense; your English is far, far better than my Italian, so if I'm not clear we'll work it out. (It also makes it harder for you, I think, as there are not so many old Chevy trucks near you! Out here there are many in the junkyards, etc.)

-- A

 

[cerryblazer]

yes my reservoir assembly is not same as LMC image 'cause it has a plastic reservoir on top of the metal master cylinder as you told me.
the plastic reservoir has 2 plastic hoses putted inside in the 2 holes of master cylinder through 2 grommets.
i hope to be clear, english mechanical technician is so hard!

so when i push down brake pedal one of 2 hoses of plastic reservoir rises for 1 cm without completly release.

I think it is as if one of two pistons of master cylinder not pushes oil in circuit.​

 

[dremu]

yes my reservoir assembly is not same as LMC image 'cause it has a plastic reservoir on top of the metal master cylinder as you told me.
the plastic reservoir has 2 plastic hoses putted inside in the 2 holes of master cylinder through 2 grommets.
i hope to be clear, english mechanical technician is so hard!

You are right; English is weird to begin with, and technical descriptions are even harder.

so when i push down brake pedal one of 2 hoses of plastic reservoir rises for 1 cm without completly release.

I think it is as if one of two pistons of master cylinder not pushes oil in circuit.​

One possibility is that the hydraulic lines are clogged, or one of the calipers or wheel cylinders is frozen up and can't move, so the master is pushing but nothing's happening.

I would flush the system; bleed it until you get clean fluid on all four corners. It's messy, and either takes two people or somewhat specialized tools, but it's simple and cheap, and doesn't require parts that you have to order from overseas.

I'd still listen around the booster for problems, see if it's the hose or valve (cheap) or the booster (expensive.)

It may be that there's something wrong, which made the master cylinder go bad, or it may be that the master cylinder just wore out, and either way, that may have worn out the booster.

-- A

 

[cerryblazer]

great!
surely i must change disks and pads but i hope the problem can be just brake hoses (to change too)
the first thing is flush liquid system, i'll check out calipers and cylinder calipers.
maybe a caliper repair kit could be a good first thing to buy from LMC truck with disks, pads and brake hoses.

 

[dremu]

Well, here (where we can give the old ones back as "cores") it's so cheap to replace calipers, there's no point in rebuilding them.

Given your shipping issues ... maybe.

-- A

 

[Wildman III]

The air nise from under the dash when the truck is off is normal .
the reasvoir expanding when the peddal it depressed is normal by gm standerds
Also wyh in the hell would you use used brake pads .
brake hoses and cailpers are notorious prolbems on chevys you also need to check the rotor thickness if they are below spec they can chrush under the pressure of the cailper . I did that once NOBRAKES!! it sucked I took out a fence and a tree stopped me

 

[sweetk30]

i had a 83 k10 with cruise control i removed. had bad air /vacume leak under dash after that. swaped 2 brake boosters out still there.

found i forgot to remove and plug the brake cut off vac dump switch for cruise control. so when i hit the brakes i was hearing a big vac leak under the dash . removed and pluged at motor port and all was perfect.

check if you might have one of these.

 

[cerryblazer]

i have not cruised control.