I have a 75 which I converted the front to disc brakes using 79 fj55 knuckles and OEM brake hardware. The problem is that I have to STAND on the brakes to stop. The brake booster is original but seems to be working since I notice a difference when I remove the vacuum line, even worse without vacuum to the booster. I also had to replace the master cylinder and I installed a new one from a 1991 4runner. I decided to use the 4runner master cylinder based on several postings by others who did the same. I also did not want to reroute brake lines and the 4runner Master was similar to the 1975 master which is much different than the 1976 disc compatible master. The rear brakes are the original drum brakes with rebuilt wheel cylinders. Do boosters go weak? How much vacuum should I have at the booster? Is the 4runner master undersized? Any other thoughts or help would be much appreciated.
Stupid brake conversion problem
- Thread starter riogreen
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- Thread starter
- #3
Calipers are new
Did you try it with the stock booster (sans residual valve)?
Do you know the 4Runner M/C bore?
When you start the truck with your foot on the brake, does the pedal drop?
Did you adjust the pushrod to match the M/C?
Do you know the 4Runner M/C bore?
When you start the truck with your foot on the brake, does the pedal drop?
Did you adjust the pushrod to match the M/C?
when I did my disc conversion on my 65 I used a 91 4 runner booster and master cylinder. And it stops really good.
- Thread starter
- #6
The original m/c was completely shot so I never tried it. I will try holding the brake as I start it to verify the booster function in the morning. The note about the rod adjustment could actually be the issue considering I adjusted that rod per the fj40 specs and not the 4runner. I will definitely look at that rod adjustment.
Thanks
Thanks
Per Fj40 specs should be fine. Sounds like your booster is on the way out. 40 ones are NLA, but 4runner, 60, 70, 80 ones will all work. Some boosters require an external check valve, but ATE make some... Volvos, VWs, and many other vehiclec use them... As well as the donor vehicles.
- Thread starter
- #8
So I checked the booster and it seems to be okay since it passed a series of test. The only test I did not complete was a vacuum gauge to verify I have 18 psi. However, I put my finger on the manifold vacuum port and the vacuum feels strong sucking my finger firmly against the port. I also checked the pushrod and it appears to have about 1/4 of play per all the specs I have found. Finally I checked the check valve and it seems fine as well.
The pedal has very little motion or travel and is very stiff at the end of its short motion. There is no spongy feeling whatsoever. From what I recall before the restoration 4 years ago the pedal had a lot more travel but also required pumping, bad m/c.
Any other ideas?
The pedal has very little motion or travel and is very stiff at the end of its short motion. There is no spongy feeling whatsoever. From what I recall before the restoration 4 years ago the pedal had a lot more travel but also required pumping, bad m/c.
Any other ideas?
If the pedal is rock hard , check those flex lines as one may have collapsed , or could be a smashed steel line somewhere .
Sarge
Sarge
I went through a similar problem recently. Swapped in an 80 booster and it helped, I then swapped in an 80 master and had awesome braking with even finger pressure on the brake pedal.
Do you have acces to a different booster?
Do you have acces to a different booster?
what's easier? applying the brakes with booster vacuum disconnected or with the vacuum connected?
- Thread starter
- #13
The 4runner M/C has a 1 inch bore which is equivalent to the original 75 M/C. I am starting to think the M/C I purchased at O’Reilly’s might be bad. Could a flawed M/C residual valve be causing my problem? Would a bad M/C display issues during the fluid bleeding process?
With regards to the comment about the bad brake lines I don’t think that’s an issue. I replaced ALL of the flexible brake lines and I pulled fluid from all four corners when I bleed the system.
I do not have another booster but I might try U-pull-it next weekend if I am still trying to resolve this issue.
I am going to get a vacuum gauge tonight and verify the vacuum from the manifold.
When the vacuum line is disconnected the pedal has additional reduction of movement towards the floor. It’s not a huge difference with or without the vacuum but it’s definitely noticeable.
With regards to the comment about the bad brake lines I don’t think that’s an issue. I replaced ALL of the flexible brake lines and I pulled fluid from all four corners when I bleed the system.
I do not have another booster but I might try U-pull-it next weekend if I am still trying to resolve this issue.
I am going to get a vacuum gauge tonight and verify the vacuum from the manifold.
When the vacuum line is disconnected the pedal has additional reduction of movement towards the floor. It’s not a huge difference with or without the vacuum but it’s definitely noticeable.
- Thread starter
- #14
It looks like the 75 FJ40 booster is a single diaphragm whereas the 4runner uses a dual diaphragm booster. Does the extra diaphragm provide additional movement and power? Could this be my problem?
Have you pulled the fitting on the intake and looked for carbon build up in the fitting itself?
Check your rears too-they help a lot. . Shut engine off and roll down hill and try brakes-while still rolling start it to see if it gets easier-to test booster. drive on gravel and skid to a stop from around 10mph-dont move it and get out and look for even skid marks. The check valve is for if the engine dies-to give it a stop on stored vacuum.
- Thread starter
- #18
The FJ stops on a dime now. I picked up a 1993 4runner booster this weekend, installed it and problem solved. I am puzzled considering the original booster passed all the test indicating it was a working booster. Maybe the dual diaphragm makes a real difference.
The 4runner booster does not have a check valve from what I can tell. I think I need to install an inline check valve. When the engine is turned off the brake goes stiff instantaneously. Any suggestions?
The 4runner booster does not have a check valve from what I can tell. I think I need to install an inline check valve. When the engine is turned off the brake goes stiff instantaneously. Any suggestions?
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The dual diaphragm makes it significantly more powerful.
Correct, needs one inline, we always snag one off of the 4Runner when robbing the booster.
ATE makes an inline one. It's used in series II VWs, Volvos, and others. A few bucks on EBay, or at the wreckers.
They Toyota dealership couldn't figure out which vehicle they came on... So I found one at the VW wreckers.
They Toyota dealership couldn't figure out which vehicle they came on... So I found one at the VW wreckers.
