how hard is it to change a rear transmission seal in a 1979 chevy truck with a small block engine?

take off drive shaft, use seal puller or small chisel to remove seal, pound new one in, re install driveshaft. check fluid level and drive. probably around 100.00 at shop?

you can do it yourself, but shop charges vary by geographical location. is it 2WD or 4WD? your driveshaft is held in place with 7 bolts at the rear driveshaft, they’re 7/16″ inch. use a socket, set the ebrake or put chokes under the tires. remove the bolts, use a prybar to pull the u-joint from the differential pinion. remove the driveshaft. it just pulls right out of the tailshaft. take a screwdriver and a hammer and pry the old seal out, then use a seal installer or equivalent to install the new seal. installation is reverse of removal.

A relative piece of cake.

remove drive shaft.

Remove the old seal (you can look right at it).

Using a light hammer and board gently drive new seal in. If you distort it, it is ruined.

though i agree w/ most said here,, they do make seal pullers and dont try to tap the new seal in place,, get a coinciding spline( the splined front u-joint holder from the junk yard cut off does works most of the time) and use it to seat the new seal

Pretty good answers going on this… As to where the seal is, it’s at the very rear of the transmission “tail housing.” Where the drive shaft connects to the trans.

A question back to you is where the oil is leaking. Look at where you park, and see if you can get a handle on where the leak is, rather than just replacing seals. You can put a piece of cardboard under the car to see the location, and then figure out what’s right above the major drips. Pan gaskets and dipstick tube o-rings will also leak. So will cooler lines, and the cooler element in the radiator. If you’re not sure where the fluid is going, best to find out.

You can get a seal seating tool – looks like a plastic dome. You set the seal (using a little Permatex Aviation Form-A-Gasket or other paint-on gasket sealer around the outside of the new seal – the metal bit, not the rubber that rides on the drive shaft. Tap it into place with a medium hammer – patience is better than main force. You may even be able to borrow the tool from a place like Auto-Zone or Kragen or such.

Be sure to mark the drive shaft relative to the yoke on the rear end. The drive shaft is externally balanced, and I’m frankly not sure why, but all the GM and Ford shop manuals I have say to be sure to get the drive shaft back in the same relative position with the rear axle input shaft.

These seals will dry out and leak over time. But so will the pan gasket, and it may gravity feed to the tail of the trans. Make sure it is the rear (output) seal. The tail housing or output seal isn’t a big deal to replace, but handling the drive shaft is a bit of a bother.

Also, there’s an output shaft seal under the tail housing, right at the back of the trans, if I remember right. If you’re not sure, or not comfortable doing this, it is best to find a shop you are comfortable with. For this sort of thing, I don’t go to a transmission shop. I’m getting old, and don’t like heaving 4wd parts around while laying on my back under the car, so I take it to a good general mechanic I know. Since this (probably) isn’t going to be a major internal transmission repair, a good general shop will treat you best.

Not too hard. Raise the rear of the vehicle to provide adequate clearance for access to the underside of the vehicle. Support with suitable jack stands for safety. Place a suitable container under theseal area to catch lubricating oil or transmission fluid that may drain from the transmission tail-shaft housing during the replacement procedure. Disconnect the drive-shaft at the universal joint located near the differential unit. Do not remove bearing caps from the universal joint. Make certain the caps cannot come off by securing with duct tap wrapped around both caps several times. If the vehicle has a two piece drive-shaft with a support bearing near the midpoint of the drive-shaft, remove the bolts (usually 2) that attach the bearing support to the frame member. Move the drive-shaft toward the rear of the vehicle until the front drive-shaft yoke comes completely out of the transmission tail-shaft. Clean any accumulated dirt, grit and road debris fron the driveshat yoke. Oil or fluid may drain from the tailshaft at this time. Catch it in the container mentiond earlier. Carefully pry out the seal taking care not to scratch the mating surface of the seal housing. Make certain that this surface is clear of dirt and grit. lubricate the new seal. Apply thick grease to the part of the seal that looks like it has a spring it. Lubricate the inside lip of the seal. Apply any reccomended sealant or lubricant to the outer circumference of the new seal. Start the seal squarely in the tailshaft housing and use a suitable tool to carefully tap the seal into the opening until it is completely seated in the housing. Apply a coating of lubricating oil to the driveshaft yoke that fits inside the tailshaft. Carefully guide the driveshaft yoke back into the tailshaft of the transmission. You may have to rotate the drive-shaft slightly to align the splines. Gently slide the driveshaft yoke into the tailshaft housing of the transmission. Reassemble the universl joint at the differential. You may have to place the transmission in neutral to allow alignment of the universal joint. Be careful not to allow the bearing cups of the urniversal joint to come off during reassembly. Torque the fasteners to the recommended value. If applicable, realign the attaching bolts of the bearing support and torque the bolts to the correct tighness. Visually check to be certain that you have reassemled everything. Make sure the transmission has the proper amount of oil or fluid. If it is a manual transmission this level is usually adjusted by removing a filler plug fron the side of the transmission and slowly adding the required amount ot the recommended lubricant to the transmission until the oil is at the level of the filler hole. It will overflow when the correct amout of oil has been added. Replace the filler plug and tighten. This must be done from the underside of the vehicle on manual transmission vehicles. Remove the safety supports and lower the vehicle. If the transmission is automatic check the fluid level now. The repair is complete now. Test drive the vhicle and check for the proper fluid levels. Check for leaks. Good luck.
Cost? It depends on the shop performing the repair. Call or visit several shops and compare estimates to determine the best value. Keep in mind that cheapest is not always the best value. Consider the competency of the technicians performing the repair. If you can, get a glance of the repair shop. Is it in disarray with tools laying about, are there puddles of oil or a noticable amount of debris on the floor? Are parts scattered about? This may not be the shop you want to repair your vehicle. A competent shop will usually have and orderly appearance. Of course there are exeptions to almost any guidelines; you must use you own good judgement in this matter. Again, good luck. Hope this information has been helpfull.

it,s not hard to change. the front 1 is what,s hard. all you do is pull the drive shaft and take a screwdriver, tear the old 1 out but watch you don,t scratch the seal surface, oil the new seal up and pound the new seal in. make sure you get it in straight. put the drive shaft back in and your good to go.