clutch goes soft after boost

Nitr0EngiE

Nitr0EngiE

Greenie N00B Member
So i have been chasing and issue with my clutch. After doing a hit in boost my clutch goes almost to floor and engagement drops really low. (Spec 3+ Sprung Clutch)

I already had a revised slave so i replaced master and the same issue was there.

Then i put a brand new slave with ate brake fluid and bleed the hell out of it. And i have the same issue. The bleeding and new parts are not working.

This has been going on about 4 weeks. I noticed there is no check valve in brake booster line so there might be one internal somewhere. Maybe that valve is bad and i should try putting one in line to prevent boost from getting into brake booster.

My clutch has no slipping and this seems hydraulic related and i called spec and they suggest hydraulic issue.

I'm not familiar with brake booster anatomy but I'm running out of easy things to replace before looking at the clutch itself....

So are there any suggestions on this or a reason why brake booster cannot cause this.

I searched and searched for possibilities in mazda community found nothing useful so i even checked other platforms and did not find anything revealing as they all only comment on salve and master issues, no brake booster mentioned.
 
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On my old 240SX with SR20DET swap I had a somewhat similar problem except that it was the brake pedal instead of the clutch and it would get rock hard with no braking power. Only did it after a WOT run at full boost. It did end up being the brake booster.

I know that's not entirely similar, but with the Speed having a shared fluid reservoir between the clutch and brakes I'm not sure what else may be shared regarding vacuum/boost routing with the brake booster. One of the local's Speed6 had a bad booster, but it presented similarly to how my 240SX did.

I'm sorry that's not very informative, but it's the best I can offer. As I was reading your post my first thought was brake booster being the issue. I'm typically not a fan of just throwing parts at a problem, but it seems like you've already followed a well thought out diagnostic and replacement plan. Booster seems like the next logical thing to me, but I just can't provide any hard evidence as to why.
 
yes, that is where Im at with it. Your booster was probably designed different and the boost affected it different or a different part of yours was bad.

If I re bleed the clutch it works perfect and its fine until I go into boost again so I am thinking there is just an internal check valve that could not withstand all the boost. This started when i started getting my car tuned and doing full pulls so I think something gave out. I do not feel it is necessary to pull the whole booster and replace it so I think I will try to install a check valve to remedy the problem.

unless someone has already had this experience there might be many people suffering from this and if it works installing a valve would be a good precaution for everyone as it cant possibly cause any harm. The brake boost on a NA car sees no boost and would never had this issue, but with boost going directly to the booster without a check valve makes me think I can add one and prevent it.

There is already a connection next to HPFP that I can undo the barb union and replace with check valve and I so happen to just order a 3/8 check valve for something else I don't really need it for so I might just see if it fits when i get home and get it on.

I called a transmission shop for an opinion and he agreed with my conclusion and was surprised i was having issues after replacing both master and slave so he suggested removing the brake boost vacuum line and running it without the booster to and retest to see if I have the same issue, but the problem with this is safety as without the booster my brakes wont work right and to re create the issue I have to go into full boost which means I will have to abruptly stop after and I need 100% brakes....... Soooooo.... this means I would be safer just trying the check valve I already have rather than risk it.
[doublepost=1469192323][/doublepost]I just did some research on a brake booster and it sounds like diaphragms and balloons inside and there is no actual connection between vacuum and fluid itself. This means that if there is a leak it may not be a valve it could be the diaphragms which means I would have to replace the whole unit, but then if there is no direct vacuum contact with the fluid then how would pressure get into brake reservoir? I will never know without cutting one of ours open and see whats in it.

I should still be safest just trying a check valve that will still allow vacuum but not boost for testing.
 
I have a similar issue, my clutch feels off after a WOT pull but goes back to normal rather quickly...curious to hear what you find out. Good luck!
 
I thought I read somewhere that these cars have in-line check valves that are sort of "hidden" within the vacuum line for the brake booster. Perhaps that's why you don't see it...

I'm not sure how much that piece of vacumm line costs, but it might be worth swapping in a new one before you go replacing the booster itself.
 
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That is excellent news, I have a check valve that will hold 100 psi that is inline I am going to try and if the stock one is failed then this one will resolve my issue.
 
SharksInSpace said:
I have a similar issue, my clutch feels off after a WOT pull but goes back to normal rather quickly...curious to hear what you find out. Good luck!
Click to expand...
^^^^^ This
After a WOT pull (which is basically a high-torque event), my clutch pedal effort increases for a little while afterwards before returning to normal. I guess it's because I'm tuned right at around the stock clutch's holding capacity.
 
I had this issue with the stock clutch before I replaced it. At the time it was thought to be the throw out bearing which is usually replaced when you do a new clutch. Did you replace the throw out bearing?
 
I would disconnect the vacum line and cap both ends to see if that is the problem before you install the check valve. And yes the stock line does have an internal check valve.

Im not sure how a vacum/boost issue with the brakes would impact the clutch though

What motor mounts do you have and are they in good condition? The motor could be moving under high torque and screwing something up.
 
I got jbr tmm su pmm and stage 2 rear mount my motor does not move.

Its def a clutch system issue. i can see if the brake booster has positive pressure it can push fluid into reservoir which can only go into the clutch system as there is no vent on the reservoir which is my theory.

I don't see a reason to not try the check valve as it will still allow vacuum. I don't want to test this in boost without proper brakes.
 
So i tried the check valve fix and it made no change so that rules out the brake booster as the issue.

So brand new fluids
New slave
New master
Extra check valve to brake booster and any time i boost the clutch goes to shit.

What makes no sense is that bleeding the clutch restores it to normal so it really acts like hydraulic.

So the next question would be what kind of clutch do i get. i want stronger than spec 3+ and i have a fidanza flywheel.
 
Wouldn't a pin hole still show fluid leaking. My stock clutch dose this same thing. Just bleed it last weekend trying to get it to stop.
 
Not if the pinhole is before the master cylinder, I'd think...maybe. It's just a thought.
 
true, but you would think that you would have a heck of a time bleeding it.
Of course, normal driving doesn't present the issue, only heavy loads. I'm saving for a clutch just due to mileage and this symptom. But op has swapped most if not all of it out and it still persists.
 
Why would the brake booster have any impact on this issue? The shared brake fluid reservoir between the brake master and clutch master cylinders only has one job, which is to supply fluid (via atmospheric pressure/gravity) to the cylinders. I would put money on a throwout bearing or pressure plate issue. The only question in my mind is why in the world it would only happen after going WOT.
 
thats my thought, but hey let them rule it out. Saves me some headache. Every time i make a pass at the track i have the same issue. i figue i just need to replace the clutch after 93k miles.
 
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