z brake problem

[datozed]

 

Have upgraded my brakes.. Front - 4 Spot callipers - Hi lux

Rear - Disc conversion using pintara discs and callipers

Mastercylinder - Upgraded from 7/8 to 15/16

 

 

The problem .. The brakes are very aggressive the require very light pressure to work but when the car has stopped and the motor is idleing the brake pedal goes rock hard giving you the feeling the car will roll away.

 

How do I stop the brakes from being so aggressive? And why is my brake pedal going rock hard when motor is idling?

 

Thanks.

[zr240]

I might be wrong but if the go rock hard it means there is somthing wrong with the brake booster. Check if you have the vacum valve around the right way.

 

Ash

[datozed]

I also thought it was a vacuum problem because the pedal goes rock hard Ive changed the valve and have had my brake booster checked for leaks and was assured that there was no problem with the brake booster. I also checked it with a vacuum gauge and also looks fine to me  What next? Thanks ???

[Toecutter]

Not sure if this will help as I'm still to convert mine, with all the bits sitting in a box ready to go but I believe the 15/16 280ZX MC has a plastic valve in it that needs to be removed when used with rear discs.

 

Refer to this write up Lurch did as that is were I got it from...

 

http://www.viczcar.com/forum/index.php/topic,2304.0.html

 

Sulio

[Zedman240®]

Hey Sulio, When you go to rear disc, you shouldn't have to touch the 280ZX cylinder; it's already setup for rear disc. That's what you do on the 7/8 260 cylinder. What you should check is the length of the rod that goes from the booster to the piston of the master cylinder. Measure it 100 times to make sure it wont go in too far and partially apply the brakes with no pressure on the pedal. I think at idle there is maximum vacuum available for the booster.

[datozed]

Thanks for the replies

  regarding the 7/8 MC have heard this information about removal of the valve on the rear break when changing to discs I do not understand why this is needed when you look in the manual and the front discs also have the same valve fitted in the MC

and as mentioned the 15/16 MC should already be adaptable to discs as the 280zx are already running them....brakes a very responsive when driving requiring very minimal pressure pedal is only hard when vehicle is idling .There is absolutely NO sponge in the pedal yet the pedal is about 100ml from the floor . Any more suggestions?

[Toecutter]

ah thanks fo rthe correction Dimitri, thats what happens when you don't read through an entire thread. Anyhow I hope you find the problem soon as I want to install my new brakes and MC in the coming weeks and knowing my luck I'll have the same issue's no doubt!

 

Sulio

[Zedman240®]

Datozed, have you checked the length of that adjustable rod? Also while the engine is idling, and the pedal rock hard, are the brakes actually on? have you tried to drive (crawl I mean)in 1st to see if they are on?

[NZeder]

There is a little valve in the rear t-piece under the car in the rear of a 240z (not sure about the 260z) this is were the prop valve is and this needs to be removed as it will keep line pressure to the rear brakes which is required for the drum setup but not with a disc setup. Well this is what we have found on the 240z were have converted to discs on the rear. The valve was removed from the inside t-piece and the brakes were better (a tricky job be worth the effort - or you could replace with a t-piece that already has no valve in it).

[tbscobraZ]

From what I have heard datsun's are very prone to getting things stuck holding the brake pedal up maybe check something isn't catching. It might just be a coincidence that it happens at idle.

[datozed]

zedman: Have checked and adjusted rod length when i installed cylinder it needed to be legthend: About 5mm. when idling with brakes on...hard pedal let brakes off and car will roll.  nzeder: My prop valve is located on firewall  brake balance feels okay.  Fronts will lock before the back.  While driving at speed 100 km and then braking once off the brake there seems to be no brake drag.

 

Question: I'm running a fairly lumpy cam, I don't know what specs (Duration and overlap) I'm thinking I'm not getting enough vacuum because of over-lap.  Any of you guys running cams come across this? 

 

Thanks.   

[nat0_240_chevZ]

Thanks for the replies

  regarding the 7/8 MC have heard this information about removal of the valve on the rear break when changing to discs I do not understand why this is needed when you look in the manual and the front discs also have the same valve fitted in the MC

and as mentioned the 15/16 MC should already be adaptable to discs as the 280zx are already running them....brakes a very responsive when driving requiring very minimal pressure pedal is only hard when vehicle is idling .There is absolutely NO sponge in the pedal yet the pedal is about 100ml from the floor . Any more suggestions?

 

did you check the fr and rear lines where the connect to the MC???

i did a swap for my mate years ago and it took a while to rrealise but when the mc change was done the MC had the F and R labelled back to front as compared to the old one. we then gently had to bend the 2 lines to get them to go to the right port and all was well, also had to adjust the pushrod between the mc and pedal  inbetweeen the booster and mc not on the pedal!!!