brake guru's need your help!

[Ryan]

Hey all, hoping someone can tell me where i've gone wrong- recently got my rear calipers rebuilt and replaced the master cylinder as we had no brakes, bled everything up today and cant seem to get any feel in the pedal, there's no more air coming out the front or the rear, yet the pedal goes to the floor and picks up bout half an inch before hitting the floor..

with the engine off i can pump the pedal and get it to hold bout halfway down with solid pressure, but once the cars started it goes to the floor again?
its a vn v6, GQ master cylinder and GQ disk brakes if it helps, have checked all vac lines and found no leaks anywhere on anything... help?

Cheers

Ryan

[GUtripper]

Is the diaphram in the brake booster shot?
Had a car with similar problems, replaced booster and all was good.

[Ryan]

u mean inside the big black part that is up against the firewall? will check that out in the morning and let you know, has to be something similar, as the master cylinder's new, the calipers are new, and the lines dont leak lol.. so theres not much left to replace

[simkell]

I would start with bleeding the master cylinder, then re-bleed the system, starting at l/h/r, r/h/r moving to the front, l/h/f, r/h/f.
this should eliminate air in the system. If the master cylinder has not been bleed on installation, it can trap air, and cause the problems you are getting.

[Ryan]

will give that a go also, is there a trick to bleeding the master cylinder? as i dont have any bleeders that i know of, it runs from the master, down the steel lines one to front diff one to rear diff, the only bleed nipples that we found were on the calipers.

[DUDELUX]

On your calipers, are the bleed nipples at the top or the bottom??
Might sound like a silly question, but when I did the brake upgrade on the DL, I hgad the calipers on the wrong sides(bleed bipples were at the bottom), and I was getting a similar thing happening to what youve described.
Check that the bleed nipples are at the top.
If thats all good, the Id be looking at the pushrod from the booster into the master cyl.

[80'sman]

if the booster diaphragm is buggered 9 times out of 10 you will hear it hiss if your engine is running and you push your pedal in.

[RAY185]

Fairly good chance you've still got air in the system if you did not bench bleed the master before fitting. Google "bench bleeding a master cylinder" should get you some info. Same idea as bleeding wheels bleed each of the ports 1 at a time by using your fingers to block and unblock the holes while pumping the piston with a screwdriver or similar implement. A bench vice and a drip tray is handy. Do that then re-bleed the lines following the correct sequence for your vehicle.

[chimpboy]

Is the master cyl sealed against the booster?

I agree with the comment that you may still have air in the system. Many a time I have been sure I had bled brakes fully but ended up getting them working after a few more bleeding attempts.

[Ryan]

thanks for the replies guys, inbetween rain storms today i managed to check the booster, its not split anywhere, bled the master cylinder, re bled the brakes, still no go but didnt get a whole lot of time between rain, im thinking there has to be air there somewhere also, might pick a nite this week and give it another full bleed,

after bleeding the master cylinder it did make a slight improvement, but the pedal's still down almost on the floor, thats why im thinking now that thats made a slight difference there has to be air elsewhere..

hope so as theres not much left to change

thanks again for the help

Cheers

Ryan

[MADSHORT]

u may have a faulty master cylinder
i put brand new one in a 80 series and it failed straight away
spoke to brake centre and they said it does hapen
cheers

[hiluxmad]

try chacking the line at the master.
had this prob recently with a lux. let some fluid out under presure and it came good....weird. apparently some hilux masters do this.

[turbo gu]

Get some plastic line clamps and clamp off the rear brake line and see if that improves the feel of the pedal.
If it does the fault is in the rear calpiers. If no improvement try clamping off the other lines

[horndog]

Does it have a brake proporting valve still, i know i had to bleed that as well to get my brakes to work. then again could have been the way i bled the brakes in the first place.

[its aford not a nissan]

my brakes were getting spongey and all the bleeding in the world didnt help, i found that the piston in the caliper was retracting when i took my foot off the brake, seems the rubber seal around the piston was flexing and not sliding as it should , and it ment that the piston on each wheel had to travel about 1.5mm before putting any pressure on the pads,

i ended up just taking out the pads on one wheel at a time , pressing the brake pedal to push the piston out further than it normally travels, refitted the pads and pushed the piston in so it was a tightish fit , did this to all the wheels and now my brakes are back to normal

its only a temp fix but i already put new kits through all the calipers to no avail

[Ryan]

tried everything we can think of, minus clamping off the lines (will give that a go later in the week) failing that can anyone recommend a good brake shop that does call outs somewhere near richmond nsw??
mite cost a bit extra but at least they will be fixed lol

[MADSHORT]

can you get hold of another master cylinder to try as i said earlier mine done exactly the same as yours
brake centre replaced it and all was good
cheers
Gordon

[Ryan]

can you get hold of another master cylinder to try as i said earlier mine done exactly the same as yours
brake centre replaced it and all was good
cheers
Gordon

will give it a shot, failing that its up to the pro's lol tuff trucks getting too close and they are driving me slightly insane trying to get them working lol

will let you know how they go

Cheers

Ryan

[bede fab]

hay mate the best way i have found to bleed brakes is clear line onto bleeder hose 400 long into a botle with bottem of line covered by fluid crack bleeder than pump peadle 7-8 times lock up bleeder note hose must be a good fit on bleeder ,also to check booster pump peadle hold down and start eng if peadle goes down booster is ok ,failing you have no leaks if the calipars you fitted may use more fluid than old drum there for you may not be getting enough head pressure to get a good peadle feel may need bigger master cylinder

[Articulator]

mate are u running gq front and rear calipers.I was running gq fronts on the rear of mine had exactly the same problem piston was to big master cylinder would not push enough fluid changed to smaller piston calipers on rear all good perfect pedal was starting to worry as well with tt looming just around the corner just a thought

[DIRTY ROCK STAR]

must be a brake sabotage happening.. or we all suck.
i noticed on the weekend my brakes were poor.
so looking at new calipers before TTC as they may have siezed.

[paulv1818]

I take it you've checked that all the pads are in right? I've taken all mine out (on a falcon) to find one was not quite in right causing a soft pedal.
long shot - front wheel bearings ok? movement in the hub will cause the pads not to seat propertly and will flex causing a soft pedal. happend in my dads gq (found out after replacing the master cylinder that didn't actually need to be replaced

[Ryan]

update- seems the issue is in the rear brakes. as with the rear brake line clamped off the pedal's sweet, got a little trick to try in the morning will let you know how i go if it works mite help a few others in the future... main prob that's been guessed is theres a pressure loss in the rear due to the length of brake line from master to the rear calipers.
Solution is a one way valve (will get correct name tomorro when i havent had so many beer's in me)
goes between the steel chassis line and the rubber line to the diff, and keeps the rear half "charged"
fingers crossed it helps

will let you's know tomorro sometime

Thanks for the help so far

Cheers

Ryan

[chimpboy]

main prob that's been guessed is theres a pressure loss in the rear due to the length of brake line from master to the rear calipers.

Did the brakes work before the recent repairs?

[its aford not a nissan]

have you checked if the pistons in the calipers are retracting when you take your foot off the brake?, as it seems this is what is happening,

you cant get a pressure drop unless there is air in the system or the piston has to travel further than normal

[Kramer]

wouldn't a one way valve just leave the rear brakes always on after you have stepped on the pedal?

[turbo gu]

update- seems the issue is in the rear brakes. as with the rear brake line clamped off the pedal's sweet, got a little trick to try in the morning will let you know how i go if it works mite help a few others in the future... main prob that's been guessed is theres a pressure loss in the rear due to the length of brake line from master to the rear calipers.
Solution is a one way valve (will get correct name tomorro when i havent had so many beer's in me)
goes between the steel chassis line and the rubber line to the diff, and keeps the rear half "charged"
fingers crossed it helps

will let you's know tomorro sometime

Thanks for the help so far


Cheers

Ryan

It wont have anything to do with the lenght of line!!!!

Make sure you got a master cylinder to suit front and rear discs not front discs and rear drums if you had it changed. There should be a residule pressure valve for the discs brakes, drum brakes dont need them. Its not a one way valve.

If it all worked before rebuilding there would be something wrong with whats been rebuilt. So either wrong master or calipers have been stuffed up.

DL said it before but are the calipers on with the nipples up.

[Ryan]

main prob that's been guessed is theres a pressure loss in the rear due to the length of brake line from master to the rear calipers.

Did the brakes work before the recent repairs?

not overly sure hey, at some stage they had to as nothing's been modified since i bought it but i bought it with seized rear calipers..
tried a few things today tho, cant get the valve i bought to seal (and found out after the brake shop had closed :( ) so just to elimenate the rear calipers i swapped them for a spare set that i got here and the pedal is the same, apparently (im only goin off what ive been told. so may be wrong)

but not having a rear proportioning valve causes fluid to drain backwards up the line too much, and causes an ultra soft pedal as its pushing fluid over a massive distance.. so hopefully i can get an adaptor this wk to suit my brake line to the new valve and they will work.. if they dont then im lost and out of ideas... and TTC is drawing closer and closer and brakes do come in handy hehe



Cheers

Ryan

[Ryan]

It wont have anything to do with the lenght of line!!!!

Make sure you got a master cylinder to suit front and rear discs not front discs and rear drums if you had it changed. There should be a residule pressure valve for the discs brakes, drum brakes dont need them. Its not a one way valve.

If it all worked before rebuilding there would be something wrong with whats been rebuilt. So either wrong master or calipers have been stuffed up.

DL said it before but are the calipers on with the nipples up.[/quote]


Thats where we're stuck, the master cyl is to suit GQ Disk/Disk. tried 2 seperate pairs of rear calipers and pedal is the same, Residule pressure valve may be what im calling the valve that the brake shop's given me.

only thing thats been rebuilt is rear calipers and didnt notice a prob before the rebuild cause they were seized, i didnt own the car prior to that so dont know if they worked or not in the first place.. we replaced the master cyl after getting rear calipers rebuilt and having the pedal prob thinking that would be it but its not.

Checked calipers and bleed nipples are at the highest point too.

[Ryan]

Still no luck even after fitting residual pressure valve to rear-
We changed back to the old master cyl- no different

Checked pistons in calipers to see if they are pulling back inside caliper- they arent.

Clamped only rear brake's off- pedal was good.

Clamped only front brakes off- pedal was good.

put a spacer on the end of the pushrod that goes into the master cylinder and with no brakes clamped we had a bit of success (before my bodgy spacer broke) pedal was right at the top, but the spacer was too long as all 4 brakes were dragging..

Does that pretty much elimenate everything and mean if i get a longer push rod tomorrow that will reach further down inside the master cyl it "should" be the problem?

or am i way off the track?

Bit i cant get my head around is how did they work in the past... we havent modified anything except getting the rear calipers rebuilt cause they were seized, and then once they were working the pedal went down