How to change Brake Master Cylinder

Post by **Juan62** » 14 Oct 2011, 07:55

For 1998 S70 T-5M,

I have search on this forum for a write on how to replace a brake master cylinder...none listed.
Does anyone here have instructions, preferrably with pictures?

I have mushy and pedal travel near the floor. No brake leaks visible form master cylinder or brake lines.
Current Replacement Completed in 2010 and 2011:
New rotors and pads on all fours.
New/rebuilt Front calipers (both).
New braided brake lines on all fours.
New Fluid Dot 4 (3 times)...did not fix my brake pedal issue. In 2010, I had front driver brake caliper sticking Pedal was normal travel when applied braking before replacements. Then, I replaced both front calipers, and pads/rotors/brake lines/fluid on all fours. Pedal travel decreased (equal or beyond gas pedal when applying brakes.

I will also replace/exchange the rear calipers, (still original OEM 1998) to be fair to the brake system and peace of mind.
Thanks.

VolvoTurbo850 · Post by **VolvoTurbo850** » 14 Oct 2011, 08:20

I would suggest you replace any defective calipers first.

I would first take a solution of soap and water and spray it around the big black cylinder. Have someone press the brakes while the engine is running. If bubbles are coming out of the brake booster it is defective. You may also notice because the engine is running with a miss because of the brake booster causing a vacuum leak.

My experience with this is that if it is mushy it might be the master over the booster. Removing the master is relatively easy because it is two bolts holding it on the booster and two lines that goes to the abs box.

If it is the booster that will be more fun and time. You have to disconnect 3 brake lines, three bolts off the ABS unit ( only have to remove the front one.. the other two can be loosened so you can slide the unit out.), disconnect the abs connection.

You then have to move to the inside of the car and release the pin to the brake pedal, and the four bolts connecting the brake booster on the firewall ( have to move the insulation aside to get to 3 of the four usually).

Some people may suggest you do not have to unscrew the brake cables but i do it to prevent damage to them and also found they are not as bad as those connections near the calipers because they are not exposed to the elements.

The main thing is that i would put the car up on the hoist or stands before proceeding, and remove the tires. This is so you can bleed the system because believe me.. your brakes will not work!

Post by **jmmxc** » 15 Oct 2011, 00:40

I have mushy and pedal travel near the floor. No brake leaks visible form master cylinder or brake lines

My experience with this is that if it is mushy it might be the master over the booster. Removing the master is relatively easy because it is two bolts holding it on the booster and two lines that goes to the abs box.

Volvoturbo850,
Great write up. Particularly the caveat at the end.
Obviously Juan62; You did a lot work on the brakes and hydraulics. My question is did you correctly bleed the system? By correctly I mean bleeding, starting RR tire, LR, FR and FL tire, as the majority of people recommends. Reason for my question? Well, 2 weeks ago, my brakes pedal was very mushy and dangerously low, with the pedal slowly going to the floor after car has stopped.
Had all rotors replaced by a friend. No success. Then I took it to nearest Monroe and was diagnosed as Master cylinder (some small leak?). I was told they need to replace master cyl. then bleed the brakes etc...$$$
Long story short, Took the car home, decided to change brake fluid and bleed the system methodically purging all the air bubbles out of each wheel, about 6 times each. After that, the mushy peadal problem went away, solved._May you should also consider bleeding the brakes according to this method if you did not do so, after you fix your caliper, and check for leaks...
Send some comments...
Good luck.
jmmxc
Volvo850GLT, 1996

Post by **Juan62** » 15 Oct 2011, 01:13

Jmmxc;
Thanks for your advice...I will do this again after rear caliper replacement.

To answer your questions: In 2011 (March), I took my car to a local repair shop to replace all rotors, brake lines, two-front calipers, new rear pads, and new brake fluid. Serveral hours later, I stopped by the shop, and they were bleeding the system (the old fashion way)...two man method, pumping the brake pedal. At that time, I didn't know that was incorrect for Volvo's. Yes, I had great brake pedal prior to this installation, but one front cailper was sticking (diagnosed and brake light came on), and I knew it was time for pads replacement because all brake hardware was original since 1998 when the car was new. Once I learned from this forum how to bleed the system, I did exactly as the instructions (as you mentioned above)...even change the fluid to Racing Blue, So I can see the new pushing out the old fluid in each caliper. Now, it maybe possible I did NOT get all the air out, therefore, as you suggested, do it again. I used the Motive brake bleeder kit, pumped to 8 psi.
Now; I hear a click or thump sound coming from the rear, when slowly pull out the driveway and apply brake pedal (windows rolled down). It feels like the rear caliper (drivers side) is not catching. No brake light on dashboard yet, brake fluid still full capacity. No leaks visible anywhere.

When you say about 6 times each wheel, do you mean consecutive?...starting with R-Rear, bleed, stop, cap, bleed, stop, cap, etc, for about 6 times, then move to R-left and repeat method? Thanks

Post by **jmmxc** » 15 Oct 2011, 15:10

I essnetially went around twice, each time engine ON (Pump 6 times, hold down, bleed, cap) for each wheel, went around the block---. back in the driveway and repeat.
This may be extreme but I got good result, the break is now firm. In your case you have the pump, which will be easier to bleed the old and get the new in, and hopefully all the air bubbles..
Keep every posted of your findings...
Good luck.
jmmxc