1992 745 NA Brake booster swap

[rstarkie]

The brake booster started leaking (audible) when applying the brakes. That degraded to a point the brake booster was no longer assisting in stopping the car. I’m currently trying to replace the booster.

I removed booster from a 1990 740T. The mounting studs are in the correct position and the booster has the same dimensions. This car had a 1 ¼” extension bracket that the master cylinder mounted to. Unbolting that bracket makes this booster look like a direct swap. However, the rod that actuates the master cylinder piston is 1 ¼” too long. When I pull outward on the rod it hard stops (metal to metal inside the booster assembly).

I have no experience with brake boosters. There are no part numbers on either booster. Is it safe to assume that the rods can not be swapped out and trying to use this booster is a lost cause?

Thanks for any insight.

 

[dl242gt]

Yes, you are correct that you need the proper pushrod length.

 

[rstarkie]

Yes, you are correct that you need the proper pushrod length.

Thank you for your reply.

 

[DET17]

If you are seaching for replacement parts.... source a 95 940 set of brake pedals (entire assembly) and a 95 940 (or 95- 960) Black painted vacuum booster. This combo will give you the best "feel" brakes if any 7/9 series.

If you just need an early replacement, I've got a complete booster/master from a low mile 94 945T that should be a bolt in.

 

[rstarkie]

If you are seaching for replacement parts.... source a 95 940 set of brake pedals (entire assembly) and a 95 940 (or 95- 960) Black painted vacuum booster. This combo will give you the best "feel" brakes if any 7/9 series.

If you just need an early replacement, I've got a complete booster/master from a low mile 94 945T that should be a bolt in.

Thanks, i will DM you.

 

[rstarkie]

Addition probing found that the clip was quite easy to remove. Pulling on the push rod resulted in it coming out. If I find that the defective booster has a rod of the same design only shorter it may interchange. I realize it is important for the push rod to be positioned exactly as it was originally.

 

[NLRegina]

If you are seaching for replacement parts.... source a 95 940 set of brake pedals (entire assembly) and a 95 940 (or 95- 960) Black painted vacuum booster. This combo will give you the best "feel" brakes if any 7/9 series.

Out of curiosity, why exactly is this?

 

[rstarkie]

The brake booster. It is normally mounted under the hood on the firewall. The master brake cylinder mounts on the two studs shown. The vacuum hose and valve attach to the rubber grommet. The rod is activated by applying pressure to the brake pedal which in turn applies pressure to another rod on the back of the booster.

 

[NLRegina]

The brake booster. It is normally mounted under the hood on the firewall. The master brake cylinder mounts on the two studs shown. The vacuum hose and valve attach to the rubber grommet. The rod is activated by applying pressure to the brake pedal which in turn applies pressure to another rod on the back of the booster.

Not sure if you were replying to my comment above, but I don't understand what makes the 95 booster with pedals have the best brake feel. What did volvo change from earlier years?

 

[rstarkie]

Not sure if you were replying to my comment above, but I don't understand what makes the 95 booster with pedals have the best brake feel. What did volvo change from earlier years?

I'm sorry, I thought you were replying to me and I misunderstood your comment. I thought maybe you were a newbie and you were asking what it was. I don't know why it would be better than previous pedal assemblies.

 

[NLRegina]

I'm sorry, I thought you were replying to me and I misunderstood your comment. I thought maybe you were a newbie and you were asking what it was. I don't know why it would be better than previous pedal assemblies.

haha all good
I'm hypothesizing that they have the linkage/rod set up to have less travel before the pedal engages, but Im no expert. One of the things I'm not super fond of on my 740 is how far I have to push to engage the brakes. No matter how many times I've bled them it's just like that I guess.

 

[rstarkie]

haha all good
I'm hypothesizing that they have the linkage/rod set up to have less travel before the pedal engages, but Im no expert. One of the things I'm not super fond of on my 740 is how far I have to push to engage the brakes. No matter how many times I've bled them it's just like that I guess.

I have heard other people mention this before. If I remember correctly it was after changing out the master cylinder. People suggested that perhaps the replacement master had a somewhat different "home" position for the piston or the master was a bit different. It was suggested that they adjust the push rod. This is my first foray into the booster territory, but I noticed the push rod length is adjustable. You can see that at the end of the rod in my photos.

I am not suggesting that you adjust it, However if the push rod is not seated against the piston there would be some travel before it made contact.

 

[DET17]

Out of curiosity, why exactly is this?

In short, the engineers changed the mechanical ratio of the pedal to the booster push rod. Years ago I had a picture of them side by side (my nice 95 pedal assembly and the stock 92 I had). The crank arm off the pivot shaft bolt had a different length as well, from memory.

There was an ancient thread in Performance, written by a British Bloke..... he mentioned that Volvo had received many complaints about the "brake feel" on the facelifted 7 series and the 9's. They finally listened to customer feedback and redesigned it for the last run of 95-98 940's (most of which North America never got).

 

[NLRegina]

Interesting - I don't know if you intended some meaning here, but you mentioned that these issues plagued the facelifted 740s and 940s. Do the early 740s have better pedal/booster setups as well?

 

[DET17]

I have zero knowledge of the early 7's. My journey began when I towed home a dead 940T sedan.

I've stated many times that I made a single point upgrade to the old Cheap Thrills sedan.... the 95+ pedals and booster/servo, and the 302mm brake rotors with 850 calipers. I cannot say which gave me what part.... but the brakes on my sedan, post upgrade, felt like a modern vehicle.... not the disappointing original version. The only hitch with the 302mm rotors is you must run a minimum of a 16" rim to clear them.

 

[rstarkie]

haha all good
I'm hypothesizing that they have the linkage/rod set up to have less travel before the pedal engages, but Im no expert. One of the things I'm not super fond of on my 740 is how far I have to push to engage the brakes. No matter how many times I've bled them it's just like that I guess.

This is just a follow up to say that eliminating the 1 1/4" extension on the booster from the 1990 740T and swapping in the pushrod from the failed 1991 740 booster worked just fine. If you ever have the opportunity to have your master cylinder off of the brake booster I found that in both instances the pushrod extends .110" beyond the cylinder mounting surface.