Brake issues - help required!

Kieronpollock

Club Member
Hi All

Looking for a fresh point of view on a brake issue with my restored 240z.

I have a new brake set up including a 1” Wilwood Master cylinder and a new 7.5” brake booster from Zcardepot, parts that came with the car as new and unused.

Car has been to an MOT tester to use their brake checking system and the cars has excellent braking pressure (attached) and is balanced properly. There are no brake fluid leaks.

The length of the push rod from booster into MC has been checked and is correct with brake pedal engaging instantly as you would expect.

Issue is the car has a stiff brake pedal, and a slight hiss can be heard when engine is running so infers there is a vacuum leak.

However, the check valve is brand new and has been checked, the booster has been pressure tested using a brand new testing kit and holds pressure.

The garage says it’s the booster at fault (on the basis of if in doubt replace it) however, it’s not easy to replace a booster especially as the original booster would have been a series 1, 1972 7” booster with the small kidney shaped bolt pattern, so hard to come by and would require refurbishment. I also don’t want to buy another Zcardepot 7.5” booster if at all possible.

I’m also concerned a 7” booster if found and refurbed may not be up for the job with the 1” Wilwood MC.

So my restorer Andy who has had many years experience on cars is a bit stumped as the stiff pedal and hiss point towards the booster but the usual tests say differently.

Any suggestions as where to look and to definitely rule out/in the booster would be helpful eg is it possible to test the other (pedal) side of the booster?

Thanks in advance
 

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jonbills

Membership Secretary
Site Administrator
Is the hissing constant with the engine running?not just when you press the brake pedal?
Have you tried to identify the source of the hissing with a stethoscope or similar?
 

Kieronpollock

Club Member
Hi

Only on brake pedal and will get Andy to identify source tomorrow. I'll also get him to re-check that there is air tight seal between MC and Brake booster

All started with brake pedal being bit too long before biting. Adjusted push rod to MC and all good but brakes felt heavy. Went to garage to check brake pressure as initally thought new pads/callipers might be the cause, all good, that's when garage heard hissing and just saids its the BB get it replaced.

My research last night regarding hissing and stiff pedal to try and assist Andy with his work on Saturday (as I'm a novice!) has told me that :
- with a 1" Wilwood MC and stock Brake booster more brake pedal force is required, the pedal is stiffer (not in a good way) and that's why people upgrade their booster to 8" - 10" ones. I can do that given location of Clutch MC on my '72 car
- reviews of the Wilwood even with bigger booster say it gives stiffer pedal and less play than other MC upgrades

I have the Zcardepot 7.5" BB and wondering should I actually be expecting a stiffer pedal vs larger BB conversions?

Then there's the dreaded Reaction Disc. I wonder if it has fallen out during fitting/adjusting as at one time the pushrod was taking out of the BB. I believe part of its role is to provide 'progression' of pedal bite as the rod in the back of BB compresses it before engaging with the MC push rod. This may have been the cause of my excessive pedal movement to start with however its 5mm thick and we only needed to adjust the push rod by less than 2mm to get the pedal to engage properly (would the Reaction Disc compress from 5mm to 2mm under braking?)

Rest of car is spot on, teething problems during initial run in pre-winter are addressed and she's all now tuned off and ready to go.....but not into the first tree!
 

Farmer42

Club Member
I would say that the stiff pedal and hissing sound are not the result of the hydraulic side of the system i.e. m/c or fluid-related but is more the result of a vacuum leak somewhere. Have you checked the vacuum pipe and connections from the inlet manifold to the booster? Even a pinhole would let the vacuum pressure off and cause the stiff pedal. There is also a valve in that pipe (see arrow on pic) that could be faulty.
 

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toopy

Club Member
The reaction disc is actually quite solid rubber, so will compress very little if at all i would of thought. I assumed it was there more as an insulator/vibration absorber than anything else.
 

toopy

Club Member
My brake booster failed a few years back, resulting in a stiff pedal rather than the usual soft pedal.
 

Faster Behr

Club Member
If you’re not getting an idle speed increase when operating the brake then I’d say it’s unlikely that you have a vacuum leak.

Have you checked the rod is not riding too tight. eg too much preload. Try dialling it back a little.

The 1” MC is shifting more weight of fluid over the same given travel distance than the smaller OEM cylinder - hence more force being required. I had a 1” clutch cylinder on my RB25 converted 260Z and it required a lead foot and it operated like an on/off switch.
 

Kieronpollock

Club Member
Thanks for this - I was wondering if my 7.5” booster is not enough to ‘offset’ the 1” MC hence the additional force. Did you just live with the RB25 pedal force?
I was also considering adding in an electric vacuum pump as they would ensure I had the booster always operating at 100% efficiency
 

Faster Behr

Club Member
I removed the 1” cylinder and got a 3/4” Wilwood Instead. It made an instant difference. There was absolutely no need for a clutch in a road car to drive like that in my opinion.

Are you sure your 1” cylinder is going to give you comfortable brake modulation with an OEM calliper/wheel cylinder setup = even if you can provide it enough assistance so as to reduce effort?
 

Kieronpollock

Club Member
when I bought the car as a rolling shell it already had a brake upgrade - Toyota calipers on front and discs on back along with the 1” MC and 7” BB. The PO’s plan was to track the car and whilst I won’t be doing that as I’ve gone in a lightly different (and far more expensive🥴) direction I’m stuck with the brake set up. I now believe the PO didn’t appreciate (as I also didn’t as a Z newbie) that whilst the 1” MC was necessary to provide enough pressure for the upgraded brakes the Zcardepot 7” BB wouldn’t be up for the job.
It’s a ‘72 car and the collective understanding is the 8.5” BB won’t fit (different bolt pattern, clutch MC position) however I found a post last night from a guy in the US who got it to fit (albeit very tightly) on a ‘72 by a bit of judicious tapping of the BB lip on the MC side. I therefore may go that route if the measurements do indeed work as the cost of a new MC (if it indeed does fit) would be less that buying a electric rotary (as opposed to the noisier piston style) vacuum pump which would ensure I get 100% vacuum efficiency but that still might not be enough in terms of overall pressure.
 

Kieronpollock

Club Member
Brake issue sorted.

Replaced the new 7” brake booster with a 8.5” one (despite Mr Google saying it won’t fit on a ‘72 car), now with the 1” MC, larger front discs and calipers and rear disc conversion everything works as it should.
It’s now ‘trackable’ as per my original plan but given expenditure on and quality of the rest of the car I think I’ll just stick to B roads 😂
Complete 7” booster set up now for sale
 
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